https://bugs.llvm.org/show_bug.cgi?id=41761
Bug ID: 41761
Summary: Missed optimization: store coalescing
Product: new-bugs
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: cos...@gmail.com
CC: htmldevelo...@gmail.com, llvm-bugs@lists.llvm.org
LLVM coalesces consecutive byte loads into larger loads when appropriate, but
doesn't do so for stores. This is unfortunate -- having these optimizations
would allow me to write platform-independent code for loading/storing
{little,big}-endian integers. Without the optimizations, I need to write an
explicit fast path for little-endian platforms.
The details below are also in the Godbolt example at
https://godbolt.org/z/45S0ID
The following functions get optimized to single mov instructions on x864_64.
uint32_t DecodeFixed32(const char* ptr) noexcept {
const uint8_t* buffer = reinterpret_cast<const uint8_t*>(ptr);
return ((static_cast<uint32_t>(buffer[0])) |
(static_cast<uint32_t>(buffer[1]) << 8) |
(static_cast<uint32_t>(buffer[2]) << 16) |
(static_cast<uint32_t>(buffer[3]) << 24));
}
uint64_t DecodeFixed64(const char* ptr) noexcept {
const uint8_t* buffer = reinterpret_cast<const uint8_t*>(ptr);
return ((static_cast<uint64_t>(buffer[0])) |
(static_cast<uint64_t>(buffer[1]) << 8) |
(static_cast<uint64_t>(buffer[2]) << 16) |
(static_cast<uint64_t>(buffer[3]) << 24) |
(static_cast<uint64_t>(buffer[4]) << 32) |
(static_cast<uint64_t>(buffer[5]) << 40) |
(static_cast<uint64_t>(buffer[6]) << 48) |
(static_cast<uint64_t>(buffer[7]) << 56));
}
However, the following functions do not get optimized to mov instructions.
void EncodeFixed32(char* dst, uint32_t value) noexcept {
uint8_t* buffer = reinterpret_cast<uint8_t*>(dst);
buffer[0] = static_cast<uint8_t>(value);
buffer[1] = static_cast<uint8_t>(value >> 8);
buffer[2] = static_cast<uint8_t>(value >> 16);
buffer[3] = static_cast<uint8_t>(value >> 24);
}
void EncodeFixed64(char* dst, uint64_t value) noexcept {
uint8_t* buffer = reinterpret_cast<uint8_t*>(dst);
buffer[0] = static_cast<uint8_t>(value);
buffer[1] = static_cast<uint8_t>(value >> 8);
buffer[2] = static_cast<uint8_t>(value >> 16);
buffer[3] = static_cast<uint8_t>(value >> 24);
buffer[4] = static_cast<uint8_t>(value >> 32);
buffer[5] = static_cast<uint8_t>(value >> 40);
buffer[6] = static_cast<uint8_t>(value >> 48);
buffer[7] = static_cast<uint8_t>(value >> 56);
}
GCC versions 8 and above optimize stores. Lower versions only optimize loads.
MSVC does not optimize loads or stores.
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