https://llvm.org/bugs/show_bug.cgi?id=31162
Bug ID: 31162
Summary: LLVM (Clang) failed to optimized little-endian integer
composition
Product: new-bugs
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: peetluge...@yandex.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Clang/LLVM is unable to optimize this:
uint32_t read32le(const uint8_t* ptr) {
return (static_cast<uint32_t>(ptr[0]) & 0x000000FFu) |
((static_cast<uint32_t>(ptr[1]) << 8) & 0x0000FF00u) |
((static_cast<uint32_t>(ptr[2]) << 16) & 0x00FF0000u) |
((static_cast<uint32_t>(ptr[3]) << 24) & 0xFF000000u);
}
LLVM code:
read32le(unsigned char const*):
movzx eax, byte ptr [rdi]
movzx ecx, byte ptr [rdi + 1]
shl ecx, 8
or ecx, eax
movzx edx, byte ptr [rdi + 2]
shl edx, 16
or edx, ecx
movzx eax, byte ptr [rdi + 3]
shl eax, 24
or eax, edx
ret
GCC (7) code:
read32le(unsigned char const*):
mov eax, DWORD PTR [rdi]
ret
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