[X86] Fix logic for optimizing movmsk(bitcast(shuffle(x))); PR67287

Prior logic would remove the shuffle iff all of the elements in `x`
where used. This is incorrect.

The issue is `movmsk` only cares about the highbits, so if the width
of the elements in `x` is smaller than the width of the elements
for the `movmsk`, then the shuffle, even if it preserves all the elements,
may change which ones are used by the highbits.

For example:
`movmsk64(bitcast(shuffle32(x, (1,0,3,2))))`

Even though the shuffle mask `(1,0,3,2)` preserves all the elements, it
flips which will be relevant to the `movmsk64` (x[1] and x[3]
before and x[0] and x[2] after).

The fix here, is to ensure that the shuffle mask can be scaled to the
element width of the `movmsk` instruction. This ensure that the
"high" elements stay "high". This is overly conservative as it
misses cases like `(1,1,3,3)` where the "high" elements stay
intact despite not be scalable, but for an relatively edge-case
optimization that should generally be handled during
simplifyDemandedBits, it seems okay.

(cherry picked from commit 1684c65bc997a8ce0ecf96a493784fe39def75de)

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 426e3143ac9b212f485d19645feafce7b45a5228..0f1cb5f1e236656c538dc751d5b9394a8147216e 100644 (file)
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -48539,13 +48539,28 @@ static SDValue combineSetCCMOVMSK(SDValue EFLAGS, X86::CondCode &CC,
   }
 
   // MOVMSK(SHUFFLE(X,u)) -> MOVMSK(X) iff every element is referenced.
-  SmallVector<int, 32> ShuffleMask;
+  // Since we peek through a bitcast, we need to be careful if the base vector
+  // type has smaller elements than the MOVMSK type.  In that case, even if
+  // all the elements are demanded by the shuffle mask, only the "high"
+  // elements which have highbits that align with highbits in the MOVMSK vec
+  // elements are actually demanded. A simplification of spurious operations
+  // on the "low" elements take place during other simplifications.
+  //
+  // For example:
+  // MOVMSK64(BITCAST(SHUF32 X, (1,0,3,2))) even though all the elements are
+  // demanded, because we are swapping around the result can change.
+  //
+  // To address this, we check that we can scale the shuffle mask to MOVMSK
+  // element width (this will ensure "high" elements match). Its slightly overly
+  // conservative, but fine for an edge case fold.
+  SmallVector<int, 32> ShuffleMask, ScaledMaskUnused;
   SmallVector<SDValue, 2> ShuffleInputs;
   if (NumElts <= CmpBits &&
       getTargetShuffleInputs(peekThroughBitcasts(Vec), ShuffleInputs,
                              ShuffleMask, DAG) &&
       ShuffleInputs.size() == 1 && !isAnyZeroOrUndef(ShuffleMask) &&
-      ShuffleInputs[0].getValueSizeInBits() == VecVT.getSizeInBits()) {
+      ShuffleInputs[0].getValueSizeInBits() == VecVT.getSizeInBits() &&
+      scaleShuffleElements(ShuffleMask, NumElts, ScaledMaskUnused)) {
     unsigned NumShuffleElts = ShuffleMask.size();
     APInt DemandedElts = APInt::getZero(NumShuffleElts);
     for (int M : ShuffleMask) {

diff --git a/llvm/test/CodeGen/X86/movmsk-cmp.ll b/llvm/test/CodeGen/X86/movmsk-cmp.ll
index f7ba49ce0e1273270393deec22695812a218f4cd..278a6a8b128eb50ea84abf99295d7cecbee253c1 100644 (file)
--- a/llvm/test/CodeGen/X86/movmsk-cmp.ll
+++ b/llvm/test/CodeGen/X86/movmsk-cmp.ll
@@ -4458,13 +4458,14 @@ define i32 @PR39665_c_ray_opt(<2 x double> %x, <2 x double> %y) {
 define i32 @pr67287(<2 x i64> %broadcast.splatinsert25) {
 ; SSE2-LABEL: pr67287:
 ; SSE2:       # %bb.0: # %entry
-; SSE2-NEXT:    movl $3, %eax
-; SSE2-NEXT:    testl %eax, %eax
-; SSE2-NEXT:    jne .LBB97_2
-; SSE2-NEXT:  # %bb.1: # %entry
 ; SSE2-NEXT:    pand {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm0
 ; SSE2-NEXT:    pxor %xmm1, %xmm1
 ; SSE2-NEXT:    pcmpeqd %xmm0, %xmm1
+; SSE2-NEXT:    pshufd {{.*#+}} xmm0 = xmm1[1,0,3,2]
+; SSE2-NEXT:    movmskpd %xmm0, %eax
+; SSE2-NEXT:    testl %eax, %eax
+; SSE2-NEXT:    jne .LBB97_2
+; SSE2-NEXT:  # %bb.1: # %entry
 ; SSE2-NEXT:    movd %xmm1, %eax
 ; SSE2-NEXT:    testb $1, %al
 ; SSE2-NEXT:    jne .LBB97_2