| Issue |
167260
|
| Summary |
Failure to reassociate uniform vector transformations with variable shuffles
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
WalterKruger
|
Since shuffles only change the arrangement of elements within a vector, it should be legal to reassociate operations that transform all lanes in the same manner to occur before/after they are shuffled even if the shuffle pattern isn't known at compile time.
This optimization isn't performed when the indexes are variable. For example, `shuffle(POW_OF_2_LUT, idx) + 1` is implemented naively rather then offsetting the table:
```asm
powOf2P1LUT_clang:
pand xmm0, xmmword ptr [rip + .LCPI0_0]
movdqa xmm1, xmmword ptr [rip + .LCPI0_1]
pshufb xmm1, xmm0
pcmpeqd xmm0, xmm0
psubb xmm1, xmm0
movdqa xmm0, xmm1
ret
```
https://godbolt.org/z/Y78Ws6Tdv
Note that this isn't legal for shuffles that can set lanes to specific constants, or that select between two vectors if only one is transformed.
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