string-equal-ignore-case: new function

* lisp/cedet/semantic/complete.el (semantic-collector-calculate-completions):
  Use `string-prefix-p' instead of explicit `compare-strings'.
* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns):
  Add `string-equal-ignore-case'.
* lisp/emacs-lisp/cl-extra.el (cl-equalp): Use `string-equal-ignore-case'.
* lisp/emacs-lisp/shadow.el (load-path-shadows-find): Likewise.
* lisp/emacs-lisp/shortdoc.el (string): Add `string-equal-ignore-case'.
* lisp/files.el (file-truename): Use `string-equal-ignore-case'.
(file-relative-name): Likewise.
* lisp/gnus/gnus-art.el (article-hide-boring-headers):
  Use `string-equal-ignore-case' instead of `gnus-string-equal'.
* lisp/gnus/gnus-util.el (gnus-string-equal):
  Remove, use `string-equal-ignore-case' instead.
* lisp/international/mule-cmds.el (describe-language-environment):
  Use `string-equal-ignore-case'.
(locale-charset-match-p): Likewise.
* lisp/man.el (Man-softhyphen-to-minus): Use `string-prefix-p'.
* lisp/minibuffer.el (completion--string-equal-p):
  Remove, use `string-equal-ignore-case' instead.
(completion--twq-all): Use `string-equal-ignore-case'.
(completion--do-completion): Likewise.
* lisp/net/browse-url.el (browse-url-default-windows-browser):
  Use `string-prefix-p' instead of explicit `compare-strings'.
* lisp/org/ob-core.el (org-babel-results-keyword):
  Use `string-equal-ignore-case' instead of explicit `compare-strings'.
(org-babel-insert-result): Likewise.
* lisp/org/org-compat.el (string-equal-ignore-case):
  Define unless defined already.
(org-mode-flyspell-verify): Use `string-equal-ignore-case'.
* lisp/org/org-lint.el (org-lint-duplicate-custom-id): Likewise.
* lisp/org/ox.el (org-export-resolve-radio-link): Use
  `string-equal-ignore-case' and `string-clean-whitespace'.
* lisp/progmodes/flymake-proc.el
  (flymake-proc--check-patch-master-file-buffer):
  Use `string-prefix-p' instead of explicit `compare-strings'.
* lisp/progmodes/idlwave.el (idlwave-class-or-superclass-with-tag):
  Use `string-equal-ignore-case' instead of explicit `compare-strings'.
* lisp/subr.el (member-ignore-case): Use `string-equal-ignore-case'.
(string-equal-ignore-case): Compare strings ignoring case.
* lisp/textmodes/bibtex.el (bibtex-string=): Remove.
(bibtex-format-entry, bibtex-font-lock-url, bibtex-autofill-entry)
(bibtex-print-help-message, bibtex-validate, bibtex-validate-globally)
(bibtex-clean-entry, bibtex-completion-at-point-function, (bibtex-url):
  Use `string-equal-ignore-case' instead of `bibtex-string='.
* lisp/textmodes/sgml-mode.el (sgml-get-context):
  Use `string-equal-ignore-case' instead of explicit `compare-strings'.
(sgml-calculate-indent): Likewise
* test/lisp/subr-tests.el (string-comparison-test):
  Add tests for `string-equal-ignore-case'.

26 files changed, 104 insertions, 124 deletions

diff --git a/doc/lispref/hash.texi b/doc/lispref/hash.texi
index d3ae673d44d..25a56bd7151 100644
--- a/doc/lispref/hash.texi
+++ b/doc/lispref/hash.texi
@@ -324,15 +324,13 @@ the same integer.
 compared case-insensitively.
 
 @example
-(defun case-fold-string= (a b)
-  (eq t (compare-strings a nil nil b nil nil t)))
-(defun case-fold-string-hash (a)
+(defun string-hash-ignore-case (a)
   (sxhash-equal (upcase a)))
 
-(define-hash-table-test 'case-fold
-  'case-fold-string= 'case-fold-string-hash)
+(define-hash-table-test 'ignore-case
+  'string-equal-ignore-case 'string-hash-ignore-case)
 
-(make-hash-table :test 'case-fold)
+(make-hash-table :test 'ignore-case)
 @end example
 
   Here is how you could define a hash table test equivalent to the
diff --git a/doc/lispref/strings.texi b/doc/lispref/strings.texi
index cb9019daa9b..bf61bb7c479 100644
--- a/doc/lispref/strings.texi
+++ b/doc/lispref/strings.texi
@@ -560,6 +560,11 @@ Representations}.
 @code{string-equal} is another name for @code{string=}.
 @end defun
 
+@defun string-equal-ignore-case string1 string2
+@code{string-equal-ignore-case} compares strings ignoring case
+differences, like @code{char-equal} when @code{case-fold-search} is
+@code{t}.
+
 @cindex locale-dependent string equivalence
 @defun string-collate-equalp string1 string2 &optional locale ignore-case
 This function returns @code{t} if @var{string1} and @var{string2} are
diff --git a/etc/NEWS b/etc/NEWS
index a31c50a850c..7c1462ee573 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2502,6 +2502,9 @@ abbrevs.  This has been generalized via the
 'save-some-buffers-functions' variable, and packages can now register
 things to be saved.
 
+** New function 'string-equal-ignore-case'.
+This compares strings ignoring case differences.
+
 ** Themes
 
 ---
diff --git a/lisp/cedet/semantic/complete.el b/lisp/cedet/semantic/complete.el
index cd04cf86434..436ad08c5fc 100644
--- a/lisp/cedet/semantic/complete.el
+++ b/lisp/cedet/semantic/complete.el
@@ -1011,20 +1011,14 @@ Output must be in semanticdb Find result format."
 			   (oref obj last-prefix)))
 	 (completionlist
 	  (cond ((or same-prefix-p
-		     (and last-prefix (eq (compare-strings
-					   last-prefix 0 nil
-					   prefix 0 (length last-prefix))
-					  t)))
+		     (and last-prefix (string-prefix-p last-prefix prefix t)))
 		 ;; We have the same prefix, or last-prefix is a
 		 ;; substring of the of new prefix, in which case we are
 		 ;; refining our symbol so just re-use cache.
 		 (oref obj last-all-completions))
 		((and last-prefix
 		      (> (length prefix) 1)
-		      (eq (compare-strings
-			   prefix 0 nil
-			   last-prefix 0 (length prefix))
-			  t))
+		      (string-prefix-p prefix last-prefix t))
 		   ;; The new prefix is a substring of the old
 		   ;; prefix, and it's longer than one character.
 		   ;; Perform a full search to pull in additional
diff --git a/lisp/emacs-lisp/byte-opt.el b/lisp/emacs-lisp/byte-opt.el
index 5705b2a8fd7..3f4af44051c 100644
--- a/lisp/emacs-lisp/byte-opt.el
+++ b/lisp/emacs-lisp/byte-opt.el
@@ -1451,7 +1451,7 @@ See Info node `(elisp) Integer Basics'."
 	 radians-to-degrees rassq rassoc read-from-string regexp-opt
          regexp-quote region-beginning region-end reverse round
 	 sin sqrt string string< string= string-equal string-lessp
-         string> string-greaterp string-empty-p
+         string> string-greaterp string-empty-p string-equal-ignore-case
          string-prefix-p string-suffix-p string-blank-p
          string-search string-to-char
 	 string-to-number string-to-syntax substring
diff --git a/lisp/emacs-lisp/cl-extra.el b/lisp/emacs-lisp/cl-extra.el
index 8e38df43c87..607810ee141 100644
--- a/lisp/emacs-lisp/cl-extra.el
+++ b/lisp/emacs-lisp/cl-extra.el
@@ -71,8 +71,7 @@ numbers of different types (float vs. integer), and also compares
 strings case-insensitively."
   (cond ((eq x y) t)
 	((stringp x)
-	 (and (stringp y) (= (length x) (length y))
-              (eq (compare-strings x nil nil y nil nil t) t)))
+	 (and (stringp y) (string-equal-ignore-case x y)))
 	((numberp x)
 	 (and (numberp y) (= x y)))
 	((consp x)
diff --git a/lisp/emacs-lisp/shadow.el b/lisp/emacs-lisp/shadow.el
index 2343a9b589f..da32e4564f6 100644
--- a/lisp/emacs-lisp/shadow.el
+++ b/lisp/emacs-lisp/shadow.el
@@ -128,11 +128,8 @@ See the documentation for `list-load-path-shadows' for further information."
 
             (if (setq orig-dir
                       (assoc file files
-                             (when dir-case-insensitive
-                               (lambda (f1 f2)
-                                 (eq (compare-strings f1 nil nil
-                                                      f2 nil nil t)
-                                     t)))))
+                             (and dir-case-insensitive
+                                  #'string-equal-ignore-case)))
 		;; This file was seen before, we have a shadowing.
 		;; Report it unless the files are identical.
                 (let ((base1 (concat (cdr orig-dir) "/" (car orig-dir)))
diff --git a/lisp/emacs-lisp/shortdoc.el b/lisp/emacs-lisp/shortdoc.el
index 05b3361cb3d..315afd4312b 100644
--- a/lisp/emacs-lisp/shortdoc.el
+++ b/lisp/emacs-lisp/shortdoc.el
@@ -243,6 +243,8 @@ A FUNC form can have any number of `:no-eval' (or `:no-value'),
   "Predicates for Strings"
   (string-equal
    :eval (string-equal "foo" "foo"))
+  (string-equal-ignore-case
+   :eval (string-equal-ignore-case "foo" "FOO"))
   (eq
    :eval (eq "foo" "foo"))
   (eql
diff --git a/lisp/files.el b/lisp/files.el
index bc74dfa7381..37ed796a687 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -1428,7 +1428,7 @@ containing it, until no links are left at any level.
 	    ;; If these are equal, we have the (or a) root directory.
 	    (or (string= dir dirfile)
 		(and (file-name-case-insensitive-p dir)
-		     (eq (compare-strings dir 0 nil dirfile 0 nil t) t))
+		     (string-equal-ignore-case dir dirfile))
 		;; If this is the same dir we last got the truename for,
 		;; save time--don't recalculate.
 		(if (assoc dir (car prev-dirs))
@@ -5459,21 +5459,17 @@ on a DOS/Windows machine, it returns FILENAME in expanded form."
 	     ;; Test for different drive letters
 	     (not (eq t (compare-strings filename 0 2 directory 0 2 fold-case)))
 	     ;; Test for UNCs on different servers
-	     (not (eq t (compare-strings
-			 (progn
-			   (if (string-match "\\`//\\([^:/]+\\)/" filename)
-			       (match-string 1 filename)
-			     ;; Windows file names cannot have ? in
-			     ;; them, so use that to detect when
-			     ;; neither FILENAME nor DIRECTORY is a
-			     ;; UNC.
-			     "?"))
-			 0 nil
-			 (progn
-			   (if (string-match "\\`//\\([^:/]+\\)/" directory)
-			       (match-string 1 directory)
-			     "?"))
-			 0 nil t)))))
+	     (not (string-equal-ignore-case
+		   (if (string-match "\\`//\\([^:/]+\\)/" filename)
+		       (match-string 1 filename)
+		     ;; Windows file names cannot have ? in
+		     ;; them, so use that to detect when
+		     ;; neither FILENAME nor DIRECTORY is a
+		     ;; UNC.
+		     "?")
+		   (if (string-match "\\`//\\([^:/]+\\)/" directory)
+		       (match-string 1 directory)
+		     "?")))))
 	   ;; Test for different remote file system identification
 	   (not (equal fremote dremote)))
 	  filename
diff --git a/lisp/gnus/gnus-art.el b/lisp/gnus/gnus-art.el
index 4b68a54ce81..e28d84e06fe 100644
--- a/lisp/gnus/gnus-art.el
+++ b/lisp/gnus/gnus-art.el
@@ -1939,8 +1939,8 @@ always hide."
 		 'boring-headers)))
 	     ;; Hide boring Newsgroups header.
 	     ((eq elem 'newsgroups)
-	      (when (gnus-string-equal
-		     (gnus-fetch-field "newsgroups")
+	      (when (string-equal-ignore-case
+		     (or (gnus-fetch-field "newsgroups") "")
 		     (gnus-group-real-name
 		      (if (boundp 'gnus-newsgroup-name)
 			  gnus-newsgroup-name
@@ -1954,7 +1954,7 @@ always hide."
 			  gnus-newsgroup-name ""))))
 		(when (and to to-address
 			   (ignore-errors
-			     (gnus-string-equal
+			     (string-equal-ignore-case
 			      ;; only one address in To
 			      (nth 1 (mail-extract-address-components to))
 			      to-address)))
@@ -1967,7 +1967,7 @@ always hide."
 			  gnus-newsgroup-name ""))))
 		(when (and to to-list
 			   (ignore-errors
-			     (gnus-string-equal
+			     (string-equal-ignore-case
 			      ;; only one address in To
 			      (nth 1 (mail-extract-address-components to))
 			      to-list)))
@@ -1980,13 +1980,13 @@ always hide."
 			  gnus-newsgroup-name ""))))
 		(when (and cc to-list
 			   (ignore-errors
-			     (gnus-string-equal
+			     (string-equal-ignore-case
 			      ;; only one address in Cc
 			      (nth 1 (mail-extract-address-components cc))
 			      to-list)))
 		  (gnus-article-hide-header "cc"))))
 	     ((eq elem 'followup-to)
-	      (when (gnus-string-equal
+	      (when (string-equal-ignore-case
 		     (message-fetch-field "followup-to")
 		     (message-fetch-field "newsgroups"))
 		(gnus-article-hide-header "followup-to")))
diff --git a/lisp/gnus/gnus-util.el b/lisp/gnus/gnus-util.el
index 218a4d242b2..31a275c7d05 100644
--- a/lisp/gnus/gnus-util.el
+++ b/lisp/gnus/gnus-util.el
@@ -1073,15 +1073,6 @@ ARG is passed to the first function."
 	s)
     (error string)))
 
-;; This might use `compare-strings' to reduce consing in the
-;; case-insensitive case, but it has to cope with null args.
-;; (`string-equal' uses symbol print names.)
-(defun gnus-string-equal (x y)
-  "Like `string-equal', except it compares case-insensitively."
-  (and (= (length x) (length y))
-       (or (string-equal x y)
-	   (string-equal (downcase x) (downcase y)))))
-
 (defcustom gnus-use-byte-compile t
   "If non-nil, byte-compile crucial run-time code."
   :type 'boolean
diff --git a/lisp/international/mule-cmds.el b/lisp/international/mule-cmds.el
index df1c06ec272..12896cc4b0e 100644
--- a/lisp/international/mule-cmds.el
+++ b/lisp/international/mule-cmds.el
@@ -2199,8 +2199,7 @@ See `set-language-info-alist' for use in programs."
 		    first nil))
 	    (dolist (elt l)
 	      (when (or (eq input-method elt)
-			(eq t (compare-strings language-name nil nil
-					       (nth 1 elt) nil nil t)))
+			(string-equal-ignore-case language-name (nth 1 elt)))
 		(when first
 		  (insert "Input methods:\n")
 		  (setq first nil))
@@ -2599,7 +2598,7 @@ Matching is done ignoring case and any hyphens and underscores in the
 names.  E.g. `ISO_8859-1' and `iso88591' both match `iso-8859-1'."
   (setq charset1 (replace-regexp-in-string "[-_]" "" charset1))
   (setq charset2 (replace-regexp-in-string "[-_]" "" charset2))
-  (eq t (compare-strings charset1 nil nil charset2 nil nil t)))
+  (string-equal-ignore-case charset1 charset2))
 
 (defvar locale-charset-alist nil
   "Coding system alist keyed on locale-style charset name.
diff --git a/lisp/man.el b/lisp/man.el
index 951e0ef9add..d66f63972ae 100644
--- a/lisp/man.el
+++ b/lisp/man.el
@@ -1241,8 +1241,7 @@ See the variable `Man-notify-method' for the different notification behaviors."
 (defun Man-softhyphen-to-minus ()
   ;; \255 is SOFT HYPHEN in Latin-N.  Versions of Debian man, at
   ;; least, emit it even when not in a Latin-N locale.
-  (unless (eq t (compare-strings "latin-" 0 nil
-				 current-language-environment 0 6 t))
+  (unless (string-prefix-p "latin-" current-language-environment t)
     (goto-char (point-min))
     (while (search-forward "­" nil t) (replace-match "-"))))
 
diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index bdf6d852a95..3daab8a1e8d 100644
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -634,9 +634,6 @@ for use at QPOS."
       (let ((qstr (funcall qfun completion)))
 	(cons qstr (length qstr))))))
 
-(defun completion--string-equal-p (s1 s2)
-  (eq t (compare-strings s1 nil nil s2 nil nil 'ignore-case)))
-
 (defun completion--twq-all (string ustring completions boundary
                                    _unquote requote)
   (when completions
@@ -650,7 +647,7 @@ for use at QPOS."
          (qfullprefix (substring string 0 qfullpos))
 	 ;; FIXME: This assertion can be wrong, e.g. in Cygwin, where
 	 ;; (unquote "c:\bin") => "/usr/bin" but (unquote "c:\") => "/".
-         ;;(cl-assert (completion--string-equal-p
+         ;;(cl-assert (string-equal-ignore-case
          ;;            (funcall unquote qfullprefix)
          ;;            (concat (substring ustring 0 boundary) prefix))
          ;;           t))
@@ -688,7 +685,7 @@ for use at QPOS."
                            (let* ((rest (substring completion
                                                    0 (length prefix)))
                                   (qrest (funcall qfun rest)))
-                             (if (completion--string-equal-p qprefix qrest)
+                             (if (string-equal-ignore-case qprefix qrest)
                                  (propertize qrest 'face
                                              'completions-common-part)
                                qprefix))))
@@ -696,7 +693,7 @@ for use at QPOS."
 		   ;; FIXME: Similarly here, Cygwin's mapping trips this
 		   ;; assertion.
                    ;;(cl-assert
-                   ;; (completion--string-equal-p
+                   ;; (string-equal-ignore-case
 		   ;;  (funcall unquote
 		   ;;           (concat (substring string 0 qboundary)
 		   ;;                   qcompletion))
@@ -1309,10 +1306,8 @@ when the buffer's text is already an exact match."
       ;; for appearance, the string is rewritten if the case changes.
       (let* ((comp-pos (cdr comp))
              (completion (car comp))
-             (completed (not (eq t (compare-strings completion nil nil
-                                                    string nil nil t))))
-             (unchanged (eq t (compare-strings completion nil nil
-                                               string nil nil nil))))
+             (completed (not (string-equal-ignore-case completion string)))
+             (unchanged (string-equal completion string)))
         (if unchanged
 	    (goto-char end)
           ;; Insert in minibuffer the chars we got.
diff --git a/lisp/net/browse-url.el b/lisp/net/browse-url.el
index a55aec76bfc..6713208d268 100644
--- a/lisp/net/browse-url.el
+++ b/lisp/net/browse-url.el
@@ -981,8 +981,7 @@ The optional NEW-WINDOW argument is not used."
                             ;; quotes in the MAILTO URLs, so we prefer
                             ;; to leave the URL with its embedded %nn
                             ;; encoding intact.
-                            (if (eq t (compare-strings url nil 7
-                                                       "file://" nil nil))
+                            (if (string-prefix-p "file://" url)
                                 (url-unhex-string url)
                               url)))))
 
diff --git a/lisp/org/ob-core.el b/lisp/org/ob-core.el
index 04af84d2e44..3d159ed38a9 100644
--- a/lisp/org/ob-core.el
+++ b/lisp/org/ob-core.el
@@ -136,8 +136,7 @@ used."
   :type 'string
   :safe (lambda (v)
 	  (and (stringp v)
-	       (eq (compare-strings "RESULTS" nil nil v nil nil t)
-		   t))))
+	       (string-equal-ignore-case "RESULTS" v))))
 
 (defcustom org-babel-noweb-wrap-start "<<"
   "String used to begin a noweb reference in a code block.
@@ -2435,7 +2434,7 @@ INFO may provide the values of these header arguments (in the
 		       ;; Escape contents from "export" wrap.  Wrap
 		       ;; inline results within an export snippet with
 		       ;; appropriate value.
-		       ((eq t (compare-strings type nil nil "export" nil nil t))
+		       ((string-equal-ignore-case type "export")
 			(let ((backend (pcase split
 					 (`(,_) "none")
 					 (`(,_ ,b . ,_) b))))
@@ -2446,14 +2445,14 @@ INFO may provide the values of these header arguments (in the
 					   backend) "@@)}}}")))
 		       ;; Escape contents from "example" wrap.  Mark
 		       ;; inline results as verbatim.
-		       ((eq t (compare-strings type nil nil "example" nil nil t))
+		       ((string-equal-ignore-case type "example")
 			(funcall wrap
 				 opening-line closing-line
 				 nil nil
 				 "{{{results(=" "=)}}}"))
 		       ;; Escape contents from "src" wrap.  Mark
 		       ;; inline results as inline source code.
-		       ((eq t (compare-strings type nil nil "src" nil nil t))
+		       ((string-equal-ignore-case type "src")
 			(let ((inline-open
 			       (pcase split
 				 (`(,_)
diff --git a/lisp/org/org-compat.el b/lisp/org/org-compat.el
index a65bf6f677a..085e32d6774 100644
--- a/lisp/org/org-compat.el
+++ b/lisp/org/org-compat.el
@@ -934,6 +934,14 @@ Implements `define-error' for older emacsen."
     (put name 'error-conditions
          (copy-sequence (cons name (get 'error 'error-conditions))))))
 
+(unless (fboundp 'string-equal-ignore-case)
+  ;; From Emacs subr.el.
+  (defun string-equal-ignore-case (string1 string2)
+    "Like `string-equal', but case-insensitive.
+Upper-case and lower-case letters are treated as equal.
+Unibyte strings are converted to multibyte for comparison."
+    (eq t (compare-strings string1 0 nil string2 0 nil t))))
+
 (unless (fboundp 'string-suffix-p)
   ;; From Emacs subr.el.
   (defun string-suffix-p (suffix string  &optional ignore-case)
@@ -1125,10 +1133,8 @@ ELEMENT is the element at point."
 	  (and log
 	       (let ((drawer (org-element-lineage element '(drawer))))
 		 (and drawer
-		      (eq (compare-strings
-			   log nil nil
-			   (org-element-property :drawer-name drawer) nil nil t)
-			  t)))))
+		      (string-equal-ignore-case
+		       log (org-element-property :drawer-name drawer))))))
 	nil)
        (t
 	(cl-case (org-element-type element)
diff --git a/lisp/org/org-lint.el b/lisp/org/org-lint.el
index 83c2d08a907..6d8cf3f2374 100644
--- a/lisp/org/org-lint.el
+++ b/lisp/org/org-lint.el
@@ -334,10 +334,8 @@ called with one argument, the key used for comparison."
    ast
    'node-property
    (lambda (property)
-     (and (eq (compare-strings "CUSTOM_ID" nil nil
-			       (org-element-property :key property) nil nil
-			       t)
-	      t)
+     (and (string-equal-ignore-case
+           "CUSTOM_ID" (org-element-property :key property))
 	  (org-element-property :value property)))
    (lambda (property _) (org-element-property :begin property))
    (lambda (key) (format "Duplicate CUSTOM_ID property \"%s\"" key))))
diff --git a/lisp/org/ox.el b/lisp/org/ox.el
index 55258bc79da..1bdf4dead89 100644
--- a/lisp/org/ox.el
+++ b/lisp/org/ox.el
@@ -80,6 +80,7 @@
 (require 'org-element)
 (require 'org-macro)
 (require 'tabulated-list)
+(require 'subr-x)
 
 (declare-function org-src-coderef-format "org-src" (&optional element))
 (declare-function org-src-coderef-regexp "org-src" (fmt &optional label))
@@ -4436,15 +4437,12 @@ INFO is a plist used as a communication channel.
 
 Return value can be a radio-target object or nil.  Assume LINK
 has type \"radio\"."
-  (let ((path (replace-regexp-in-string
-	       "[ \r\t\n]+" " " (org-element-property :path link))))
+  (let ((path (string-clean-whitespace (org-element-property :path link))))
     (org-element-map (plist-get info :parse-tree) 'radio-target
       (lambda (radio)
-	(and (eq (compare-strings
-		  (replace-regexp-in-string
-		   "[ \r\t\n]+" " " (org-element-property :value radio))
-		  nil nil path nil nil t)
-		 t)
+	(and (string-equal-ignore-case
+	      (string-clean-whitespace (org-element-property :value radio))
+              path)
 	     radio))
       info 'first-match)))
 
diff --git a/lisp/progmodes/flymake-proc.el b/lisp/progmodes/flymake-proc.el
index 4ab16831bc1..249ae9dff2f 100644
--- a/lisp/progmodes/flymake-proc.el
+++ b/lisp/progmodes/flymake-proc.el
@@ -399,10 +399,7 @@ instead of reading master file from disk."
                    (not (string-match (format "\\.%s\\'" source-file-extension)
                                       inc-name))
                    (setq inc-name (concat inc-name "." source-file-extension)))
-              (when (eq t (compare-strings
-                           source-file-nondir nil nil
-                           inc-name (- (length inc-name)
-                                       (length source-file-nondir)) nil))
+              (when (string-suffix-p source-file-nondir inc-name)
                 (flymake-log 3 "inc-name=%s" inc-name)
                 (when (flymake-proc--check-include source-file-name inc-name
                                                    include-dirs)
diff --git a/lisp/progmodes/idlwave.el b/lisp/progmodes/idlwave.el
index a2061fde762..b3dc3cac763 100644
--- a/lisp/progmodes/idlwave.el
+++ b/lisp/progmodes/idlwave.el
@@ -7528,7 +7528,7 @@ associated TAG, if any."
        (setq cl (pop sclasses))
        (let ((tags (idlwave-class-tags cl)))
 	 (while tags
-	   (if (eq t (compare-strings tag 0 nil (car tags) 0 nil t))
+	   (if (string-equal-ignore-case tag (car tags))
 	     (throw 'exit cl))
 	   (setq tags (cdr tags))))))))
 
diff --git a/lisp/subr.el b/lisp/subr.el
index a0ad967533d..c82b33bba53 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -868,7 +868,7 @@ Non-strings in LIST are ignored."
   (declare (side-effect-free t))
   (while (and list
 	      (not (and (stringp (car list))
-			(eq t (compare-strings elt 0 nil (car list) 0 nil t)))))
+			(string-equal-ignore-case elt (car list)))))
     (setq list (cdr list)))
   list)
 
@@ -5302,6 +5302,12 @@ and replace a sub-expression, e.g.
       (setq matches (cons (substring string start l) matches)) ; leftover
       (apply #'concat (nreverse matches)))))
 
+(defun string-equal-ignore-case (string1 string2)
+  "Like `string-equal', but case-insensitive.
+Upper-case and lower-case letters are treated as equal.
+Unibyte strings are converted to multibyte for comparison."
+  (eq t (compare-strings string1 0 nil string2 0 nil t)))
+
 (defun string-prefix-p (prefix string &optional ignore-case)
   "Return non-nil if PREFIX is a prefix of STRING.
 If IGNORE-CASE is non-nil, the comparison is done without paying attention
diff --git a/lisp/textmodes/bibtex.el b/lisp/textmodes/bibtex.el
index 333cfa51695..64cb0dc0fe6 100644
--- a/lisp/textmodes/bibtex.el
+++ b/lisp/textmodes/bibtex.el
@@ -2213,10 +2213,6 @@ Point must be at beginning of preamble.  Do not move point."
 
 ;; Helper Functions
 
-(defsubst bibtex-string= (str1 str2)
-  "Return t if STR1 and STR2 are equal, ignoring case."
-  (eq t (compare-strings str1 0 nil str2 0 nil t)))
-
 (defun bibtex-delete-whitespace ()
   "Delete all whitespace starting at point."
   (if (looking-at "[ \t\n]+")
@@ -2657,7 +2653,7 @@ Formats current entry according to variable `bibtex-entry-format'."
 
                     ;; update page dashes
                     (if (and (memq 'page-dashes format)
-                             (bibtex-string= field-name "pages")
+                             (string-equal-ignore-case field-name "pages")
                              (progn (goto-char beg-text)
                                     (looking-at
                                      "\\([\"{][0-9]+\\)[ \t\n]*--?[ \t\n]*\\([0-9]+[\"}]\\)")))
@@ -2710,7 +2706,7 @@ Formats current entry according to variable `bibtex-entry-format'."
                     ;; use book title of crossref'd entry
                     (if (and (memq 'inherit-booktitle format)
                              empty-field
-                             (bibtex-string= field-name "booktitle")
+                             (string-equal-ignore-case field-name "booktitle")
                              crossref-key)
                         (let ((title (save-excursion
                                        (save-restriction
@@ -3503,7 +3499,7 @@ If NO-BUTTON is non-nil do not generate buttons."
           (let ((lst bibtex-generate-url-list) url)
             (while (and (not found) (setq url (car (pop lst))))
               (goto-char start)
-              (setq found (and (bibtex-string= name (car url))
+              (setq found (and (string-equal-ignore-case name (car url))
                                (re-search-forward (cdr url) end t))))))
       (unless found (goto-char end)))
     (if (and found (not no-button))
@@ -3954,7 +3950,7 @@ entry (for example, the year parts of the keys)."
 	(goto-char (1- (match-beginning 0)))
 	(bibtex-beginning-of-entry)
 	(if (and (looking-at bibtex-entry-head)
-                 (bibtex-string= type (bibtex-type-in-head))
+                 (string-equal-ignore-case type (bibtex-type-in-head))
                  ;; In case we found ourselves :-(
                  (not (equal key (setq tmp (bibtex-key-in-head)))))
 	  (setq other-key tmp
@@ -3963,7 +3959,7 @@ entry (for example, the year parts of the keys)."
 	(bibtex-end-of-entry)
 	(bibtex-skip-to-valid-entry)
 	(if (and (looking-at bibtex-entry-head)
-                 (bibtex-string= type (bibtex-type-in-head))
+                 (string-equal-ignore-case type (bibtex-type-in-head))
                  ;; In case we found ourselves :-(
                  (not (equal key (setq tmp (bibtex-key-in-head))))
                  (or (not other-key)
@@ -4004,9 +4000,9 @@ interactive calls."
   (interactive (list nil t))
   (unless field (setq field (car (bibtex-find-text-internal nil nil comma))))
   (if (string-search "@" field)
-      (cond ((bibtex-string= field "@string")
+      (cond ((string-equal-ignore-case field "@string")
              (message "String definition"))
-            ((bibtex-string= field "@preamble")
+            ((string-equal-ignore-case field "@preamble")
              (message "Preamble definition"))
             (t (message "Entry key")))
     (let* ((case-fold-search t)
@@ -4588,7 +4584,7 @@ Return t if test was successful, nil otherwise."
                         bounds field idx)
                    (while (setq bounds (bibtex-parse-field))
                      (let ((field-name (bibtex-name-in-field bounds)))
-                       (if (and (bibtex-string= field-name "month")
+                       (if (and (string-equal-ignore-case field-name "month")
                                 ;; Check only abbreviated month fields.
                                 (let ((month (bibtex-text-in-field-bounds bounds)))
                                   (not (or (string-match "\\`[\"{].+[\"}]\\'" month)
@@ -4669,7 +4665,7 @@ Return t if test was successful, nil otherwise."
             (while (re-search-forward bibtex-entry-head nil t)
               (setq entry-type (bibtex-type-in-head)
                     key (bibtex-key-in-head))
-              (if (or (and strings (bibtex-string= entry-type "string"))
+              (if (or (and strings (string-equal-ignore-case entry-type "string"))
                       (assoc-string entry-type bibtex-entry-alist t))
                   (if (member key key-list)
                       (push (format-message
@@ -5046,10 +5042,10 @@ At end of the cleaning process, the functions in
 	       (user-error "Not inside a BibTeX entry")))
         (entry-type (bibtex-type-in-head))
         (key (bibtex-key-in-head)))
-    (cond ((bibtex-string= entry-type "preamble")
+    (cond ((string-equal-ignore-case entry-type "preamble")
            ;; (bibtex-format-preamble)
            (user-error "No clean up of @Preamble entries"))
-          ((bibtex-string= entry-type "string")
+          ((string-equal-ignore-case entry-type "string")
            (setq entry-type 'string))
           ;; (bibtex-format-string)
           (t (bibtex-format-entry)))
@@ -5326,10 +5322,10 @@ entries from minibuffer."
                (>= pnt (bibtex-start-of-text-in-field bounds))
                (<= pnt (bibtex-end-of-text-in-field bounds)))
           (setq name (bibtex-name-in-field bounds t)
-                compl (cond ((bibtex-string= name "crossref")
+                compl (cond ((string-equal-ignore-case name "crossref")
                              ;; point is in crossref field
                              'crossref-key)
-                            ((bibtex-string= name "month")
+                            ((string-equal-ignore-case name "month")
                              ;; point is in month field
                              bibtex-predefined-month-strings)
                             ;; point is in other field
@@ -5488,7 +5484,7 @@ Return the URL or nil if none can be generated."
             (while (and (not url) (setq scheme (pop lst)))
               ;; Verify the match of `bibtex-font-lock-url' by
               ;; comparing with TEXT.
-              (when (and (bibtex-string= (caar scheme) name)
+              (when (and (string-equal-ignore-case (caar scheme) name)
                          (string-match (cdar scheme) text))
                 (setq url t scheme (cdr scheme)))))))
 
diff --git a/lisp/textmodes/sgml-mode.el b/lisp/textmodes/sgml-mode.el
index 8f9b603ef5f..ba0a94b4a1f 100644
--- a/lisp/textmodes/sgml-mode.el
+++ b/lisp/textmodes/sgml-mode.el
@@ -1536,8 +1536,7 @@ not the case, the first tag returned is the one inside which we are."
 	    ;; [ Well, actually it depends, but we don't have the info about
 	    ;; when it doesn't and when it does.   --Stef ]
 	    (setq ignore nil)))
-	 ((eq t (compare-strings (sgml-tag-name tag-info) nil nil
-				 (car stack) nil nil t))
+	 ((string-equal-ignore-case (sgml-tag-name tag-info) (car stack))
 	  (setq stack (cdr stack)))
 	 (t
 	  ;; The open and close tags don't match.
@@ -1549,9 +1548,8 @@ not the case, the first tag returned is the one inside which we are."
 		  ;; but it's a bad assumption when tags *are* closed but
 		  ;; not properly nested.
 		  (while (and (cdr tmp)
-			      (not (eq t (compare-strings
-					  (sgml-tag-name tag-info) nil nil
-					  (cadr tmp) nil nil t))))
+			      (not (string-equal-ignore-case
+				    (sgml-tag-name tag-info) (cadr tmp))))
 		    (setq tmp (cdr tmp)))
 		  (if (cdr tmp) (setcdr tmp (cddr tmp)))))
 	    (message "Unmatched tags <%s> and </%s>"
@@ -1701,9 +1699,8 @@ LCON is the lexical context, if any."
 	    (there (point)))
        ;; Ignore previous unclosed start-tag in context.
        (while (and context unclosed
-		   (eq t (compare-strings
-			  (sgml-tag-name (car context)) nil nil
-			  unclosed nil nil t)))
+		   (string-equal-ignore-case
+		    (sgml-tag-name (car context)) unclosed))
 	 (setq context (cdr context)))
        ;; Indent to reflect nesting.
        (cond
diff --git a/lisp/vc/vc-dispatcher.el b/lisp/vc/vc-dispatcher.el
index f50d45217c7..e2a490092b5 100644
--- a/lisp/vc/vc-dispatcher.el
+++ b/lisp/vc/vc-dispatcher.el
@@ -761,8 +761,7 @@ the buffer contents as a comment."
 ;;     (while (and (not member) fileset)
 ;;       (let ((elem (pop fileset)))
 ;;         (if (if (file-directory-p elem)
-;;                 (eq t (compare-strings buffer-file-name nil (length elem)
-;;                                        elem nil nil))
+;;                 (string-prefix-p elem buffer-file-name)
 ;;               (eq (current-buffer) (get-file-buffer elem)))
 ;;             (setq member t))))
 ;;     member))
diff --git a/test/lisp/subr-tests.el b/test/lisp/subr-tests.el
index 84f3e41148d..d45f409e85b 100644
--- a/test/lisp/subr-tests.el
+++ b/test/lisp/subr-tests.el
@@ -368,6 +368,13 @@
              2)))
 
 (ert-deftest string-comparison-test ()
+  (should (string-equal-ignore-case "abc" "abc"))
+  (should (string-equal-ignore-case "abc" "ABC"))
+  (should (string-equal-ignore-case "abc" "abC"))
+  (should-not (string-equal-ignore-case "abc" "abCD"))
+  (should (string-equal-ignore-case "S" "s"))
+  ;; not yet: (should (string-equal-ignore-case "SS" "ß"))
+
   (should (string-lessp "abc" "acb"))
   (should (string-lessp "aBc" "abc"))
   (should (string-lessp "abc" "abcd"))