https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671
--- Comment #93 from Bernd Edlinger <bernd.edlinger at hotmail dot de> ---
I tried a bit more with lto
and discovered that -flto -O2 removes all the
code from test() although it is marked noinline, noclone
To work around that, I added an asm statement
initializing all these mem-refs, now the test case
looks like that:
cat t.cc
#include <cstddef>
//namespace std { enum class byte: unsigned char {}; };
struct t {
int x;
//std::byte kk[1];
unsigned char k;
} __attribute__((may_alias));
t t1;
t t2;
t *t3;
t *t4;
void __attribute__((noinline, noclone))
test()
{
t1 = t2;
*t4 = *t3;
}
int main()
{
asm("":"=m"(t1), "=m"(t2), "=m"(t3), "=m"(t4));
test();
}
This prevents -flto -O2 from removing the mem-refs.
But I discovered that the asm output arguments
don't use the alias-set zero:
g++ -flto -O2 -fdump-rtl-final t.cc
has this rtl from the asm stmt:
(insn:TI 5 2 6 2 (parallel [
(set (mem/c:DI (symbol_ref:DI ("t1") [flags 0x2] <var_decl
0x7f0d3d106ab0 t1>) [2 t1+0 S8 A64])
(asm_operands:DI ("") ("=m") 0 []
[]
[] t.cc:23))
(set (mem/c:DI (symbol_ref:DI ("t2") [flags 0x2] <var_decl
0x7f0d3d106b40 t2>) [2 t2+0 S8 A64])
(asm_operands:DI ("") ("=m") 1 []
[]
[] t.cc:23))
(set (mem/f/c:DI (symbol_ref:DI ("t3") [flags 0x2] <var_decl
0x7f0d3d106bd0 t3>) [1 t3+0 S8 A64])
(asm_operands:DI ("") ("=m") 2 []
[]
[] t.cc:23))
(set (mem/f/c:DI (symbol_ref:DI ("t4") [flags 0x2] <var_decl
0x7f0d3d106c60 t4>) [1 t4+0 S8 A64])
(asm_operands:DI ("") ("=m") 3 []
[]
[] t.cc:23))
(clobber (reg:CCFP 18 fpsr))
(clobber (reg:CC 17 flags))
]) "t.cc":23 -1
(expr_list:REG_UNUSED (reg:CCFP 18 fpsr)
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil))))
Again alias set 2 for t1, and t2.
This is identical also for non-lto build.
