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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug rtl-optimization/109483] New: Unoptimal jump threading with assembler flag output Date: Wed, 12 Apr 2023 08:54:33 +0000 [thread overview] Message-ID: <bug-109483-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109483 Bug ID: 109483 Summary: Unoptimal jump threading with assembler flag output Product: gcc Version: 13.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: rtl-optimization Assignee: unassigned at gcc dot gnu.org Reporter: ubizjak at gmail dot com Target Milestone: --- Following testcase (int3 mnemonic is for marker only): --cut here-- _Bool foo (int cnt) { if (cnt == -1) { _Bool success; asm volatile("int3" : "=@ccz" (success)); if (!success) return 0; } asm volatile ("" ::: "memory"); return 1; } --cut here-- compiles w/ -O2 on x86_64 to: 0000000000000000 <foo>: 0: 83 ff ff cmp $0xffffffff,%edi 3: 74 0b je 10 <foo+0x10> 5: b8 01 00 00 00 mov $0x1,%eax a: c3 retq b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 10: cc int3 11: 0f 94 c0 sete %al 14: 74 ef je 5 <foo+0x5> 16: c3 retq Please note setting of %al before conditional jump. The instruction could be moved after the jump, where the register could be cleared using "xor %eax, %eax", similar to what clang creates: 0000000000000000 <foo>: 0: 83 ff ff cmp $0xffffffff,%edi 3: 75 06 jne b <foo+0xb> 5: cc int3 6: 74 03 je b <foo+0xb> 8: 31 c0 xor %eax,%eax a: c3 retq b: b0 01 mov $0x1,%al d: c3 retq Also note that for ZF=1 gcc sets %al to 1, jumps to *5 where the register is again set to 1. This is not the case in the clang code.
next reply other threads:[~2023-04-12 8:54 UTC|newest] Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-04-12 8:54 ubizjak at gmail dot com [this message] 2023-04-12 8:59 ` [Bug middle-end/109483] " pinskia at gcc dot gnu.org 2023-04-12 10:47 ` [Bug tree-optimization/109483] Unoptimal uncprop " rguenth at gcc dot gnu.org 2023-04-12 10:48 ` rguenth at gcc dot gnu.org 2023-04-12 14:40 ` ubizjak at gmail dot com 2023-04-12 19:43 ` pinskia at gcc dot gnu.org
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