http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58805
--- Comment #15 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to vries from comment #14)
> > No it shouldn't. It should return true if the stmt has side-effects that
> > are _not_ explicit in the statement. This side-effect is explicitely there.
>
> Hmm, a rather deceptive name then, if gimple_stmt_has_side_effects does not
> in fact tell whether a gimple statement has side-effects or not.
> I wonder if any other use sites are mistaken in the semantics, and what
> could be a better name.
A side-effect is by definition an effect that happens "on the side", so
I think the name quite matches.
> A better name according to your description could be
> gimple_stmt_has_implicit_side_effects, but I'm not sure I understand how to
> differentiate between implicit and explicit here. Marking an asm with
> volatile, is that implicit or explicit, and why?
Well, is in a = b + c the computation of a + c a side-effect or not?
Is in a = b the assignment to 'a' a side-effect or not. Neither I would
argue.
> AFAIU, the name gimple_stmt_has_call_or_volatile_side_effects reflects what
> the implementation does.
No, for calls the side-effects of the callee body are not explicit (unless
it is pure or const).
> Anyways, this tentative patch fixes the problem in tail-merge:
> ...
> Author: Tom de Vries <t...@codesourcery.com>
> Date: Mon Oct 21 13:40:28 2013 +0200
>
> * tree-ssa-tail-merge.c (stmt_local_def): Improve side-effect check.
>
> diff --git a/gcc/testsuite/gcc.dg/pr58805.c b/gcc/testsuite/gcc.dg/pr58805.c
> new file mode 100644
> index 0000000..6e6eba5
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr58805.c
> @@ -0,0 +1,23 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -ftree-tail-merge -fdump-tree-pre" } */
> +
> +static inline void bar(unsigned long *r)
> +{
> + unsigned long t;
> + __asm__ (
> + "movq $42, %[t]\n\t"
> + "movq %[t], %[r]\n\t"
> + : [t] "=&r" (t), [r] "=r" (*r)
> + );
> +}
> +
> +void foo(int n, unsigned long *x, unsigned long *y)
> +{
> + if (n == 0)
> + bar(x);
> + else
> + bar(y);
> +}
> +
> +/* { dg-final { scan-tree-dump-times "__asm__" 2 "pre"} } */
> +/* { dg-final { cleanup-tree-dump "pre" } } */
> diff --git a/gcc/tree-ssa-tail-merge.c b/gcc/tree-ssa-tail-merge.c
> index 9094935..0090da6 100644
> --- a/gcc/tree-ssa-tail-merge.c
> +++ b/gcc/tree-ssa-tail-merge.c
> @@ -300,7 +300,8 @@ stmt_local_def (gimple stmt)
> tree val;
> def_operand_p def_p;
>
> - if (gimple_has_side_effects (stmt))
> + if (gimple_has_side_effects (stmt)
> + || gimple_vdef (stmt) != NULL_TREE)
> return false;
>
> def_p = SINGLE_SSA_DEF_OPERAND (stmt, SSA_OP_DEF);
> ...
>
> I'll test this one.
Looks good to me. Btw, you'd have had the same issue with the aggregate
return of a pure/const function call, no?
