On Mon, Sep 28, 2026 at 9:39 AM Richard Biener
<[email protected]> wrote:
>
> On Sun, Sep 27, 2026 at 9:11 PM Uros Bizjak <[email protected]> wrote:
> >
> > For an output operand whose constraint allows both a register and
> > memory (e.g. "=rm"), expand_asm_stmt only binds the operand directly
> > to the address of its underlying storage when the operand's value is
> > a bare, non-ADDR_EXPR-based MEM_REF or a plain, memory-resident DECL.
> > Every other lvalue -- most commonly a struct/union member, an array
> > element, or a complex number's real/imaginary part -- is instead
> > always routed through a fresh pseudo register that gets copied to the
> > real destination in a separate insn after the asm, even though
> > nothing about the constraint requires it.
> >
> > Extend the set of output values eligible for direct addressing to
> > also cover COMPONENT_REF (except through a bit-field, which is not
> > byte-addressable and still needs the existing register-based
> > read-modify-write handling), ARRAY_REF, ARRAY_RANGE_REF, REALPART_EXPR
> > and IMAGPART_EXPR.
>
> At least the comment needs updating, but I also wonder why we need the else {}
> and whether we can more easily put a condition on that with the if case being
> seemingly the more "default" handling?
>
> Also the DECL_P handling would need to apply to the MEM_REF && ADDR_EXPR
> case (not sure what the DECL_P handling is about - this code really
> lacks comments...).
> And I'd like to see using REFERENCE_CLASS_P (val) && (... exclusions ...)
> rather
> than enumerating parts of it that are OK.
I based the whole idea on your 2022 patch [1], where the patch
deliberately left the other tcc_reference operands unexplored. Since
my GIMPLE knowledge is limited, my patch just trivially adds some of
them (with the help of AI) to the existing condition (the ones that I
can easily check with the C examples).
Also, I noticed that the current unpatched compiler produces unoptimal code for:
int m;
int foo (void)
{
asm ("mov %%fs, %0" : "=rm" (m));
return m;
}
gcc -O2:
mov %fs, m(%rip)
movl m(%rip), %eax
ret
which would really benefit from using a temporary reg.
Uros.
