Hi,
Digging into a test case failure with section anchors, I found
dependence analysis return false negatives, leading to bad optimization
by cse1. Two variables are synthetically constructed aliases. One is
addressed relative to the section anchor and the other using a symbol
ref, yet write_dependence_p() claims that they could not alias. I can
reproduce that miscompile on aarch64, ppc64, and s390x. How is
write_dependence_p() supposed to handle such cases with section anchors?
Example: (simplified version of gcc.c-torture/execute/alias-2.c)
int off; // causing nonzero offset of a to the section anchor
int a;
extern int b __attribute__ ((alias("a")));
/* ... */
int main()
b=1; <-- uses symbol_ref to b
a=2; <-- uses the section anchor + offset 4
if (b!=2)
__builtin_abort ();
return 0;
}
so write_dependence gets to compare the canonicalized addresses
(const:DI (plus:DI (symbol_ref:DI ("*.LANCHOR0") [flags 0x182])
(const_int 4 [0x4])))
and
(symbol_ref:DI ("b") [flags 0x602] <var_decl b>)
and
I believe there are two ideas in the code, which could apply to this
scenario: (a) identify anchor+4 being equal to the symbol_ref for a (in
canonicalization?), (b) instead lookup (symbol_ref "b") -> a ->
anchor+4, identify the shared section anchor and compare offsets
(compare_base_symbol_refs appears to attempt that; yet find_base_term
has dropped a's offset to the section anchor), or (c) that I am not
aware of?
How is that analysis supposed to drill into the addresses?
(see also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92294 for the
initial report on aarch64 and my full example)
Marius
--
Marius Hillenbrand
Linux on Z development
IBM Deutschland Research & Development GmbH
Vors. des Aufsichtsrats: Gregor Pillen / Geschäftsführung: Dirk Wittkopp
Sitz der Gesellschaft: Böblingen / Registergericht: Amtsgericht
Stuttgart, HRB 243294
