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Second, the extension-dce work that was started in 2H2024. This is a new optimization pass which is designed to track liveness of the result of sign/zero extension instructions. If the bits set by a sign/zero extension are never read and never escape (implicitly read), then the extension is useless and can potentially be removed. This is expected to improve x264's scalar implementation by roughly 1%. It is also expected to be a very small improvement to the coremark benchmark. Other codes show very small improvements. This code was originally targeted at gcc-14 (Spring 2024), but it was decided to wait rather than push integration of the code well after feature freeze.
Third, eliminate the redundant sign extensions after various double→int conversions. This can likely use the same procedure as we have with the 32bit ops like addw. Jivan has taken care of this.
Fourth, eliminate the redundant sign extension after an inlined strcmp. More generally expose the sign extended nature of function return values.
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Seventh, in the extension elimination pass, handle things like (any_extend:DI (ashift: SI ...)). The extension will be replaced by a SUBREG, which we can then push into the operands creating word-sized operations. That eliminates the unnecessary extension can can help in code like this:
| Code Block |
|---|
void foo(unsigned char *data, unsigned int lo_bit) {
unsigned int mask = ((1UL << 1) - 1) << lo_bit;
*data = (*data & ~mask) | ((1 << lo_bit) & mask);
}
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One the unnecessary extension is eliminated a simple backend pattern can be used to simplify all that to a simple lb+bset+sb.
Stakeholders/Partners
RISE:
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- Note additional case we should add to ext-dce.
- Note Jivan has taken care of extensions following fp→int conversions. It will be submitted once gcc-15 is open for development.
- Note the two additional identified opportunities to remove redundant sign extensions. Change development state to in-progress.
- Added yet another idea for ABI defined extension elimination in REE.
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