On 7/27/2026 10:44 AM, Rohith Kapelli wrote:
simplify_ternary_operation folds IF_THEN_ELSE (cond, a, a) to a using
rtx_equal_p, which ignores the memory attributes. Two loads from the
same address into the same register are rtx_equal_p even when they have
incompatible alias sets, so the fold returned one arm's MEM and the
result inherited just that arm's (too narrow) alias set.
noce_try_ifelse_collapse builds exactly such an IF_THEN_ELSE from the two
sides of
long f (int a, void *cc, long *d)
{
long long c;
*d = 0;
if (a) c = *(long *) cc; else c = *(long long *) cc;
*d = 1;
return c;
}
so ce1 replaced the two loads with a single long long load; a later pass,
seeing that a long long load does not alias the long store to *d, deleted
the "dead" *d = 0, and with cc == d the function returned a stale value.
Rather than dropping the fold when the attributes differ, keep it and
return a reference that only claims what both operands guarantee, in the
spirit of merge_memattrs: alias set 0 when the sets differ, MEM_EXPR and
offset cleared when they disagree, and the minimum alignment. The
operands can be shared, so the attributes are set on a shallow copy
rather than in place.
Unlike merge_memattrs, which fixes up two references that both remain in
the instruction stream, this returns a single reference standing in for
either arm, so the size is kept only when both agree instead of taking
the larger one. BLKmode is left alone because there MEM_ATTRS describes
the size of the access itself, and the fold is also skipped when the two
operands disagree about volatility. Address spaces need no check:
rtx_equal_p already fails for MEMs in different address spaces.
Several tree passes can factor the two loads with a conservative type
before RTL and so hide this: PRE and code hoisting on the release
branches, and the phi-opt load factoring (PR125557) on trunk. The test
disables them so the if-conversion path is exercised on every affected
version. It is a live wrong-code at -O2 on the 13/14/15/16 branches --
gcc-13 miscompiles the reduced case at plain -O2 with no such flags at
all.
PR rtl-optimization/125683
gcc/ChangeLog:
* simplify-rtx.cc (simplify_context::simplify_ternary_operation):
When folding an IF_THEN_ELSE of two equal MEM operands with
different memory attributes, return a copy whose attributes are
widened to what both operands allow.
gcc/testsuite/ChangeLog:
* gcc.dg/pr125683.c: New test.
Signed-off-by: Rohith Kapelli <[email protected]>
This version is better. I think we probably have to worry about
MEM_READONLY_P. For the transformation in question if only one of the
two loads has MEM_READONLY_P, then we would drop that property on the
if-converted load. I can't immediately think of when that could happen,
but I'm willing to believe it could possibly happen through one or more
obscure transformations.
We should check MEM_NOTRAP_P and MEM_POINTER as well. I think in both
cases if there is disagreement, then we drop the flag.
I think with those fixes in place we'll be ready to integrate.
jeff
