[Modules][PCH] Hash input files content

Summary:
When files often get touched during builds, the mtime based validation
leads to different problems in implicit modules builds, even when the
content doesn't actually change:

- Modules only: module invalidation due to out of date files. Usually causing rebuild traffic.
- Modules + PCH: build failures because clang cannot rebuild a module if it comes from building a PCH.
- PCH: build failures because clang cannot rebuild a PCH in case one of the input headers has different mtime.

This patch proposes hashing the content of input files (headers and
module maps), which is performed during serialization time. When looking
at input files for validation, clang only computes the hash in case
there's a mtime mismatch.

I've tested a couple of different hash algorithms availble in LLVM in
face of building modules+pch for `#import <Cocoa/Cocoa.h>`:
- `hash_code`: performace diff within the noise, total module cache increased by 0.07%.
- `SHA1`: 5% slowdown. Haven't done real size measurements, but it'd be BLOCK_ID+20 bytes per input file, instead of BLOCK_ID+8 bytes from `hash_code`.
- `MD5`: 3% slowdown. Like above, but BLOCK_ID+16 bytes per input file.

Given the numbers above, the patch uses `hash_code`. The patch also
improves invalidation error msgs to point out which type of problem the
user is facing: "mtime", "size" or "content".

rdar://problem/29320105

Reviewers: dexonsmith, arphaman, rsmith, aprantl

Subscribers: jkorous, cfe-commits, ributzka

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67249

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374841 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 22 insertions, 0 deletions

diff --git a/include/clang/Driver/Options.td b/include/clang/Driver/Options.td
index 22676b6787..3ce6fcf29f 100644
--- a/include/clang/Driver/Options.td
+++ b/include/clang/Driver/Options.td
@@ -1368,6 +1368,28 @@ def fmodules_validate_system_headers : Flag<["-"], "fmodules-validate-system-hea
   HelpText<"Validate the system headers that a module depends on when loading the module">;
 def fno_modules_validate_system_headers : Flag<["-"], "fno-modules-validate-system-headers">,
   Group<i_Group>, Flags<[DriverOption]>;
+
+def fvalidate_ast_input_files_content:
+  Flag <["-"], "fvalidate-ast-input-files-content">,
+  Group<f_Group>, Flags<[CC1Option]>,
+  HelpText<"Compute and store the hash of input files used to build an AST."
+           " Files with mismatching mtime's are considered valid"
+           " if both contents is identical">;
+def fmodules_validate_input_files_content:
+  Flag <["-"], "fmodules-validate-input-files-content">,
+  Group<f_Group>, Flags<[DriverOption]>,
+  HelpText<"Validate PCM input files based on content if mtime differs">;
+def fno_modules_validate_input_files_content:
+  Flag <["-"], "fno_modules-validate-input-files-content">,
+  Group<f_Group>, Flags<[DriverOption]>;
+def fpch_validate_input_files_content:
+  Flag <["-"], "fpch-validate-input-files-content">,
+  Group<f_Group>, Flags<[DriverOption]>,
+  HelpText<"Validate PCH input files based on content if mtime differs">;
+def fno_pch_validate_input_files_content:
+  Flag <["-"], "fno_pch-validate-input-files-content">,
+  Group<f_Group>, Flags<[DriverOption]>;
+
 def fmodules : Flag <["-"], "fmodules">, Group<f_Group>,
   Flags<[DriverOption, CC1Option]>,
   HelpText<"Enable the 'modules' language feature">;