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| author | Björn Steinbrink <bsteinbr@gmail.com> | 2015-04-27 00:18:02 +0200 |
|---|---|---|
| committer | Björn Steinbrink <bsteinbr@gmail.com> | 2015-04-29 14:45:23 +0200 |
| commit | 36dccec2f39c7e1da7f056ea421ad5256df3fb0b (patch) | |
| tree | 7a3cf65a4e09ba2c61f5425126e254faa2e1060f /src/test/codegen/stack-alloc-string-slice.cc | |
| parent | 8f991d1fc27a176254ebfe99ed7e5a339cb9c7e2 (diff) | |
| download | rust-36dccec2f39c7e1da7f056ea421ad5256df3fb0b.tar.gz rust-36dccec2f39c7e1da7f056ea421ad5256df3fb0b.zip | |
Currently, LLVM lowers a cttz8 on x86_64 to these instructions:
```asm
movzbl %dil, %eax
bsfl %eax, %eax
movl $32, %ecx
cmovnel %eax, %ecx
cmpl $32, %ecx
movl $8, %eax
cmovnel %ecx, %eax
```
which has some unnecessary overhead, having two conditional moves.
To improve the codegen, we can zero extend the 8 bit integer, then set
bit 8 and perform a cttz operation on the extended value. That way
there's no conditional operation involved at all.
Diffstat (limited to 'src/test/codegen/stack-alloc-string-slice.cc')
0 files changed, 0 insertions, 0 deletions