| Issue |
81765
|
| Summary |
[X86] Improve KnownBits for X86ISD::PSADBW nodes
|
| Labels |
backend:X86
|
| Assignees |
|
| Reporter |
RKSimon
|
X86TargetLowering::computeKnownBitsForTargetNode currently just sets the upper 48-bits to be zero and makes no attempt to determine the remaining active bits.
```cpp
typedef uint8_t __v2qu __attribute__((__vector_size__(2)));
auto sum_of_bits(__m128i x) {
x = _mm_and_si128(x, _mm_set1_epi8(1));
x = _mm_sad_epu8(x, _mm_setzero_si128());
return __builtin_convertvector(x, __v2qu);
}
```
https://godbolt.org/z/74bYbTMh8
```asm
sum_of_bits(long long vector[2]): # @sum_of_bits(long long vector[2])
pand .LCPI0_0(%rip), %xmm0 # v16i8 values: 0-1
pxor %xmm1, %xmm1
psadbw %xmm0, %xmm1 # v2i64 values: 0-8
pand .LCPI0_1(%rip), %xmm1 # unneccessary
packuswb %xmm1, %xmm1
packuswb %xmm1, %xmm1
packuswb %xmm1, %xmm1
movd %xmm1, %eax
retq
```
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