From b60bc913cca7439d29a7ec9e9a7f448d8841b43c Mon Sep 17 00:00:00 2001 From: Jakub Jelinek Date: Sat, 19 Mar 2022 13:53:12 +0100 Subject: [PATCH] i386: Don't emit pushf;pop for __builtin_ia32_readeflags_u* with unused lhs [PR104971] __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. 2022-03-19 Jakub Jelinek PR middle-end/104971 * config/i386/i386-expand.cc (ix86_expand_builtin) : If ignore, don't push/pop anything and just return const0_rtx. * gcc.target/i386/pr104971.c: New test. --- gcc/config/i386/i386-expand.cc | 3 +++ gcc/testsuite/gcc.target/i386/pr104971.c | 18 ++++++++++++++++++ 2 files changed, 21 insertions(+) create mode 100644 gcc/testsuite/gcc.target/i386/pr104971.c diff --git a/gcc/config/i386/i386-expand.cc b/gcc/config/i386/i386-expand.cc index e85641d..2272854 100644 --- a/gcc/config/i386/i386-expand.cc +++ b/gcc/config/i386/i386-expand.cc @@ -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 diff --git a/gcc/testsuite/gcc.target/i386/pr104971.c b/gcc/testsuite/gcc.target/i386/pr104971.c new file mode 100644 index 0000000..80ac6b6 --- /dev/null +++ b/gcc/testsuite/gcc.target/i386/pr104971.c @@ -0,0 +1,18 @@ +/* PR middle-end/104971 */ +/* { dg-do run } */ +/* { dg-options "-O2" } */ + +#include + +__attribute__((noipa)) void +foo (void) +{ + __readeflags (); +} + +int +main () +{ + foo (); + return 0; +} -- 2.7.4