https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87984
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |rguenth at gcc dot gnu.org
--- Comment #16 from Richard Biener <rguenth at gcc dot gnu.org> ---
The "issue" of GIMPLE is that while it treats local reg vars as "memory":
# .MEM_11 = VDEF <.MEM_2>
a = 1;
# VUSE <.MEM_11>
__asm__("add %1, %0" : "=g" o_16 : "r" a, "0" o_13);
it doesn't "properly" translate asm constraints mentioning only architectural
register names to alias that "memory:
__asm__ __volatile__("xor %%eax, %%eax" : : : "eax");
(this one is missing a virtual operand). We'd have to consider this kind
of "registers" memory. Currently we have
/* Clobber all memory and addressable symbols for asm ("" : : : "memory");
*/
if (gimple_asm_clobbers_memory_p (stmt))
add_virtual_operand (fn, stmt, opf_def);
that would need to include walking all local and global reg vars and
architecturally comparing the underlying register. Note how
compare_base_decls "gives up" for local reg vars unless the register
specifier is byte-for-byte the same (so it can identify some must-alias
cases but never no-alias ones).
Elsewhere people have asked for better (gimple) asm optimizations with
regard to alias analysis - currently the precision is at the virtual
operand level.
