On Sat, Mar 19, 2022 at 8:39 AM Jakub Jelinek <ja...@redhat.com> wrote:
>
> Hi!
>
> __builtin_ia32_readeflags_u* aren't marked const or pure I think
> intentionally, so that they aren't CSEd from different regions of a function
> etc. because we don't and can't easily track all dependencies between
> it and surrounding code (if somebody looks at the condition flags, it is
> dependent on the vast majority of instructions).
> But the builtin itself doesn't have any side-effects, so if we ignore the
> result of the builtin, there is no point to emit anything.
>
> There is a LRA bug that miscompiles the testcase which this patch makes
> latent, which is certainly worth fixing too, but IMHO this change
> (and maybe ix86_gimple_fold_builtin too which would fold it even earlier
> when it looses lhs) is worth it as well.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> 2022-03-19 Jakub Jelinek <ja...@redhat.com>
>
> PR middle-end/104971
> * config/i386/i386-expand.cc
> (ix86_expand_builtin) <case IX86_BUILTIN_READ_FLAGS>: If ignore,
> don't push/pop anything and just return const0_rtx.
>
> * gcc.target/i386/pr104971.c: New test.
OK.
Thanks,
Uros.
>
> --- gcc/config/i386/i386-expand.cc.jj 2022-03-14 10:34:34.149924022 +0100
> +++ gcc/config/i386/i386-expand.cc 2022-03-18 18:28:25.029500676 +0100
> @@ -13556,6 +13556,9 @@ rdseed_step:
> return target;
>
> case IX86_BUILTIN_READ_FLAGS:
> + if (ignore)
> + return const0_rtx;
> +
> emit_insn (gen_push (gen_rtx_REG (word_mode, FLAGS_REG)));
>
> if (optimize
> --- gcc/testsuite/gcc.target/i386/pr104971.c.jj 2022-03-18 18:21:38.879034278
> +0100
> +++ gcc/testsuite/gcc.target/i386/pr104971.c 2022-03-18 18:21:24.300232909
> +0100
> @@ -0,0 +1,18 @@
> +/* PR middle-end/104971 */
> +/* { dg-do run } */
> +/* { dg-options "-O2" } */
> +
> +#include <x86intrin.h>
> +
> +__attribute__((noipa)) void
> +foo (void)
> +{
> + __readeflags ();
> +}
> +
> +int
> +main ()
> +{
> + foo ();
> + return 0;
> +}
>
> Jakub
>
