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random improvement for variable shift codegen. 

llvm-svn: 111813
This commit is contained in:
Chris Lattner 2010-08-23 17:30:29 +00:00
parent a73af6f698
commit a42202e0e4
1 changed files with 14 additions and 2 deletions
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16
llvm/lib/Target/X86/README-SSE.txt
View File

@ -2,8 +2,20 @@
// Random ideas for the X86 backend: SSE-specific stuff.
//===---------------------------------------------------------------------===//
- Consider eliminating the unaligned SSE load intrinsics, replacing them with
unaligned LLVM load instructions.
//===---------------------------------------------------------------------===//
SSE Variable shift can be custom lowered to something like this, which uses a
small table + unaligned load + shuffle instead of going through memory.
__m128i_shift_right:
.byte 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15
.byte -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1
...
__m128i shift_right(__m128i value, unsigned long offset) {
return _mm_shuffle_epi8(value,
_mm_loadu_si128((__m128 *) (___m128i_shift_right + offset)));
}
//===---------------------------------------------------------------------===//