On 8/8/2026 8:54 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.
MEM_READONLY_P, MEM_NOTRAP_P and MEM_POINTER each assert something about
the reference, so the copy keeps them only when both operands do. They
are rtx flag bits rather than MEM_ATTRS fields, so shallow_copy_rtx takes
them from the first operand and they have to be cleared by hand; the
equal-attributes early exit tests them too, so a disagreement in a flag
alone still goes through the copy. merge_memattrs already drops the
first two this way when it commons two references, so this only follows
it; it does not look at MEM_POINTER, which is treated the same way here
because it is an assertion about the loaded value in just the same
sense.
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. Volatility is not merged: it constrains
when the access happens rather than describing the memory, so it can be
neither weakened (that would lose a required access) nor strengthened as
merge_memattrs does (that would add a volatile access on the arm which
did not have one, there being a single access left for either arm), and
the fold is declined instead. 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.
* gcc.dg/rtl/aarch64/pr125683-flags.c: New test.
Signed-off-by: Rohith Kapelli<[email protected]>
Thanks. I've pushed this to the trunk. I'll backport to the
appropriate release branches after it's had time to simmer on the trunk.
Jeff
