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* [gcc r14-8346] rtl-optimization/113255 - base_alias_check vs. pointer difference
@ 2024-01-23 7:08 Richard Biener
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From: Richard Biener @ 2024-01-23 7:08 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:a98d5130a6dcff2ed4db371e500550134777b8cf
commit r14-8346-ga98d5130a6dcff2ed4db371e500550134777b8cf
Author: Richard Biener <rguenther@suse.de>
Date: Mon Jan 15 12:55:20 2024 +0100
rtl-optimization/113255 - base_alias_check vs. pointer difference
When the x86 backend generates code for cpymem with the rep_8byte
strathegy for the 8 byte aligned main rep movq it needs to compute
an adjusted pointer to the source after doing a prologue aligning
the destination. It computes that via
src_ptr + (dest_ptr - orig_dest_ptr)
which is perfectly fine. On RTL this is then
8: r134:DI=const(`g'+0x44)
9: {r133:DI=frame:DI-0x4c;clobber flags:CC;}
REG_UNUSED flags:CC
56: r129:DI=const(`g'+0x4c)
57: {r129:DI=r129:DI&0xfffffffffffffff8;clobber flags:CC;}
REG_UNUSED flags:CC
REG_EQUAL const(`g'+0x4c)&0xfffffffffffffff8
58: {r118:DI=r134:DI-r129:DI;clobber flags:CC;}
REG_DEAD r134:DI
REG_UNUSED flags:CC
REG_EQUAL const(`g'+0x44)-r129:DI
59: {r119:DI=r133:DI-r118:DI;clobber flags:CC;}
REG_DEAD r133:DI
REG_UNUSED flags:CC
but as written find_base_term happily picks the first candidate
it finds for the MINUS which means it picks const(`g') rather
than the correct frame:DI. This way find_base_term (but also
the unfixed find_base_value used by init_alias_analysis to
initialize REG_BASE_VALUE) performs pointer analysis isn't
sound. The following restricts the handling of multi-operand
operations to the case we know only one can be a pointer.
This for example causes gcc.dg/tree-ssa/pr94969.c to miss some
RTL PRE (I've opened PR113395 for this). A more drastic patch,
removing base_alias_check results in only gcc.dg/guality/pr41447-1.c
regressing (so testsuite coverage is bad). I've looked at
gcc.dg/tree-ssa tests and mostly scheduling changes are present,
the cc1plus .text size is only 230 bytes worse. With the this
less drastic patch below most scheduling changes are gone.
x86_64 might not the very best target to test for impact, but
test coverage on other targets is unlikely to be very much better.
PR rtl-optimization/113255
* alias.cc (find_base_term): Remove PLUS/MINUS handling
when both operands are not CONST_INT_P.
* gcc.dg/torture/pr113255.c: New testcase.
Diff:
---
gcc/alias.cc | 28 +++++-----------------------
gcc/testsuite/gcc.dg/torture/pr113255.c | 27 +++++++++++++++++++++++++++
2 files changed, 32 insertions(+), 23 deletions(-)
diff --git a/gcc/alias.cc b/gcc/alias.cc
index b2ec4806d22..6fad4b29d31 100644
--- a/gcc/alias.cc
+++ b/gcc/alias.cc
@@ -2077,31 +2077,13 @@ find_base_term (rtx x, vec<std::pair<cselib_val *,
if (tmp1 == pic_offset_table_rtx && CONSTANT_P (tmp2))
return find_base_term (tmp2, visited_vals);
- /* If either operand is known to be a pointer, then prefer it
- to determine the base term. */
- if (REG_P (tmp1) && REG_POINTER (tmp1))
- ;
- else if (REG_P (tmp2) && REG_POINTER (tmp2))
- std::swap (tmp1, tmp2);
- /* If second argument is constant which has base term, prefer it
- over variable tmp1. See PR64025. */
- else if (CONSTANT_P (tmp2) && !CONST_INT_P (tmp2))
+ if (CONST_INT_P (tmp1))
std::swap (tmp1, tmp2);
- /* Go ahead and find the base term for both operands. If either base
- term is from a pointer or is a named object or a special address
- (like an argument or stack reference), then use it for the
- base term. */
- rtx base = find_base_term (tmp1, visited_vals);
- if (base != NULL_RTX
- && ((REG_P (tmp1) && REG_POINTER (tmp1))
- || known_base_value_p (base)))
- return base;
- base = find_base_term (tmp2, visited_vals);
- if (base != NULL_RTX
- && ((REG_P (tmp2) && REG_POINTER (tmp2))
- || known_base_value_p (base)))
- return base;
+ /* We can only handle binary operators when one of the operands
+ never leads to a base value. */
+ if (CONST_INT_P (tmp2))
+ return find_base_term (tmp1, visited_vals);
/* We could not determine which of the two operands was the
base register and which was the index. So we can determine
diff --git a/gcc/testsuite/gcc.dg/torture/pr113255.c b/gcc/testsuite/gcc.dg/torture/pr113255.c
new file mode 100644
index 00000000000..2f009524c6b
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr113255.c
@@ -0,0 +1,27 @@
+/* { dg-do run } */
+/* { dg-additional-options "-mtune=k8 -mstringop-strategy=rep_8byte" { target { x86_64-*-* i?86-*-* } } } */
+
+struct S { unsigned a[10]; unsigned y; unsigned b[6]; } g[2];
+
+__attribute__((noinline, noclone)) int
+test (int x)
+{
+ struct S e[2] = { g[0], g[1] };
+ int r = 0;
+ if (x >= 0)
+ {
+ r++;
+ e[1].y++;
+ }
+ g[1] = e[1];
+ return r;
+}
+
+int
+main ()
+{
+ test (1);
+ if (g[1].y != 1)
+ __builtin_abort ();
+ return 0;
+}
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