PR rtl-opt/67609

	* config/i386/i386.c (ix86_cannot_change_mode_class): Disallow
	narrowing subregs on SSE and MMX registers.
	* doc/tm.texi.in (CANNOT_CHANGE_MODE_CLASS): Clarify when subregs that
	appear to be sub-words of multi-register pseudos must be rejected.
	* doc/tm.texi: Regenerate.
testsuite/
	* gcc.target/i386/pr67609-2.c: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@229458 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 13 insertions, 2 deletions

diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
index 69b6cf90e21..93620ebd325 100644
--- a/gcc/doc/tm.texi.in
+++ b/gcc/doc/tm.texi.in
@@ -2461,8 +2461,8 @@ in the reload pass.
 If defined, a C expression that returns nonzero for a @var{class} for which
 a change from mode @var{from} to mode @var{to} is invalid.
 
-For the example, loading 32-bit integer or floating-point objects into
-floating-point registers on the Alpha extends them to 64 bits.
+For example, loading 32-bit integer or floating-point objects into
+floating-point registers on Alpha extends them to 64 bits.
 Therefore loading a 64-bit object and then storing it as a 32-bit object
 does not store the low-order 32 bits, as would be the case for a normal
 register.  Therefore, @file{alpha.h} defines @code{CANNOT_CHANGE_MODE_CLASS}
@@ -2473,6 +2473,17 @@ as below:
   (GET_MODE_SIZE (FROM) != GET_MODE_SIZE (TO) \
    ? reg_classes_intersect_p (FLOAT_REGS, (CLASS)) : 0)
 @end smallexample
+
+Even if storing from a register in mode @var{to} would be valid,
+if both @var{from} and @code{raw_reg_mode} for @var{class} are wider
+than @code{word_mode}, then we must prevent @var{to} narrowing the
+mode.  This happens when the middle-end assumes that it can load
+or store pieces of an @var{N}-word pseudo, and that the pseudo will
+eventually be allocated to @var{N} @code{word_mode} hard registers.
+Failure to prevent this kind of mode change will result in the
+entire @code{raw_reg_mode} being modified instead of the partial
+value that the middle-end intended.
+
 @end defmac
 
 @hook TARGET_IRA_CHANGE_PSEUDO_ALLOCNO_CLASS