| Message ID | YjWInQFmlqKLsl6W@tucnak |
|---|---|
| State | New |
| Headers | show |
| Series | i386: Don't emit pushf;pop for __builtin_ia32_readeflags_u* with unused lhs [PR104971] | expand |
On Sat, Mar 19, 2022 at 8:39 AM Jakub Jelinek <jakub@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 <jakub@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 >
--- 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; +}