OpenE2K
/
gcc
2
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity

[PATCH] PR rtl-optimization/101885: Prevent combine from clobbering flags 

This patch addresses PR rtl-optimization/101885 which is a P2 wrong code
regression.  In combine, if the resulting fused instruction is a parallel
of two sets which fails to be recognized by the backend, combine tries to
emit these as two sequential set instructions (known as split_i2i3).
As each set is recognized the backend may add any necessary "clobbers".
The code currently checks that any clobbers added to the first "set"
don't interfere with the second set, but doesn't currently handle the
case that clobbers added to the second set may interfere/kill the
destination of the first set (which must be live at this point).
The solution is to cut'n'paste the "clobber" logic from just a few
lines earlier, suitably adjusted for the second instruction.

One minor nit that may confuse a reviewer is that at this point in
the code we've lost track of which set was first and which was second
(combine chooses dynamically, and the recog processes that adds the
clobbers may have obfuscated the original SET_DEST) so the actual test
below is to confirm that any newly added clobbers (on the second set
instruction) don't overlap either set0's or set1's destination.

2022-02-04  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR rtl-optimization/101885
	* combine.cc (try_combine): When splitting a parallel into two
	sequential sets, check not only that the first doesn't clobber
	the second but also that the second doesn't clobber the first.

gcc/testsuite/ChangeLog
	PR rtl-optimization/101885
	* gcc.dg/pr101885.c: New test case.
This commit is contained in:
Roger Sayle 2022-02-04 09:32:21 +00:00
parent 5b6370295d
commit 49365d511a
2 changed files with 49 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
gcc/combine.cc
View File

@ -4017,6 +4017,24 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
insn_code_number = recog_for_combine (&newpat, i3, &new_i3_notes);
/* Likewise, recog_for_combine might have added clobbers to NEWPAT.
Checking that the SET0's SET_DEST and SET1's SET_DEST aren't
mentioned/clobbered, ensures NEWI2PAT's SET_DEST is live. */
if (insn_code_number >= 0 && GET_CODE (newpat) == PARALLEL)
{
for (i = XVECLEN (newpat, 0) - 1; i >= 0; i--)
if (GET_CODE (XVECEXP (newpat, 0, i)) == CLOBBER)
{
rtx reg = XEXP (XVECEXP (newpat, 0, i), 0);
if (reg_overlap_mentioned_p (reg, SET_DEST (set0))
|| reg_overlap_mentioned_p (reg, SET_DEST (set1)))
{
undo_all ();
return 0;
}
}
}
if (insn_code_number >= 0)
split_i2i3 = 1;
}

31
gcc/testsuite/gcc.dg/pr101885.c Normal file
View File

@ -0,0 +1,31 @@
/* { dg-do run } */
/* { dg-options "-O3" } */
int a = 3, c;
short b = 5, d, f;
volatile short e;
__attribute__((noipa)) void
foo (void)
{
}
int
main ()
{
for (f = 0; f != 2; f++)
{
int g = a;
if (b)
if (a)
for (a = b = 0; b <= 3; b++)
;
for (c = 0; c != 16; ++c)
e;
}
b = d;
foo ();
if (a != 0)
__builtin_abort ();
return 0;
}