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From: "gcc at rjk dot terraraq.uk" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug other/103345] New: missed optimization: add/xor individual bytes to form a word Date: Sun, 21 Nov 2021 11:17:03 +0000 [thread overview] Message-ID: <bug-103345-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103345 Bug ID: 103345 Summary: missed optimization: add/xor individual bytes to form a word Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: other Assignee: unassigned at gcc dot gnu.org Reporter: gcc at rjk dot terraraq.uk Target Milestone: --- All code generated with godbolt's idea of 'trunk'. See https://godbolt.org/z/Wcj61PKKG Source: #include <stdint.h> uint32_t load_le_32_or(const uint8_t *ptr) { return ((uint32_t)ptr[0]) | ((uint32_t)ptr[1] << 8) | ((uint32_t)ptr[2] << 16) | ((uint32_t)ptr[3] << 24); } uint32_t load_le_32_add(const uint8_t *ptr) { return ((uint32_t)ptr[0]) + ((uint32_t)ptr[1] << 8) + ((uint32_t)ptr[2] << 16) + ((uint32_t)ptr[3] << 24); } uint32_t load_le_32_xor(const uint8_t *ptr) { return ((uint32_t)ptr[0]) ^ ((uint32_t)ptr[1] << 8) ^ ((uint32_t)ptr[2] << 16) ^ ((uint32_t)ptr[3] << 24); } The ^ version is admittedly a bit of an odd choice but the + version is a reasonably natural way to write the code. Code on gcc -O2: load_le_32_or: mov eax, DWORD PTR [rdi] ret load_le_32_add: movzx eax, BYTE PTR [rdi+1] movzx edx, BYTE PTR [rdi+2] sal eax, 8 sal edx, 16 add eax, edx movzx edx, BYTE PTR [rdi] add eax, edx movzx edx, BYTE PTR [rdi+3] sal edx, 24 add eax, edx ret load_le_32_xor: movzx eax, BYTE PTR [rdi+1] movzx edx, BYTE PTR [rdi+2] sal eax, 8 sal edx, 16 xor eax, edx movzx edx, BYTE PTR [rdi] xor eax, edx movzx edx, BYTE PTR [rdi+3] sal edx, 24 xor eax, edx ret Code on clang -O2: load_le_32_or: # @load_le_32_or mov eax, dword ptr [rdi] ret load_le_32_add: # @load_le_32_add mov eax, dword ptr [rdi] ret load_le_32_xor: # @load_le_32_xor mov eax, dword ptr [rdi] ret
next reply other threads:[~2021-11-21 11:17 UTC|newest] Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-11-21 11:17 gcc at rjk dot terraraq.uk [this message] 2021-11-21 12:44 ` [Bug tree-optimization/103345] " roger at nextmovesoftware dot com 2021-11-22 8:41 ` roger at nextmovesoftware dot com 2021-11-22 18:17 ` cvs-commit at gcc dot gnu.org 2021-11-24 8:55 ` cvs-commit at gcc dot gnu.org 2021-11-25 19:47 ` roger at nextmovesoftware dot com 2021-11-30 10:37 ` cvs-commit at gcc dot gnu.org
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