[PATCH] D113948: [DAGCombiner] Prevent unfoldMaskedMerge from creating an AND with two inverted inputs.

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.
craig.topper added reviewers: jrtc27, spatel, lebedev.ri.
Herald added subscribers: luke957, ecnelises, luismarques, pengfei, s.egerton, PkmX, simoncook, hiraditya.
craig.topper requested review of this revision.
Herald added a project: LLVM.

It's possible that the mask is already a NOT. At least if InstCombine
hasn't canonicalized the input. In that case we will form an ANDN with
X instead of with Y. So we don't need to worry about Y being a constant.

We might need to check that X isn't a constant instead, but we don't
have a test case for that yet.

This fixes a size regression found when trying to enable this combine
for RISCV in D113937 <https://reviews.llvm.org/D113937>.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D113948

Files:
  llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  llvm/test/CodeGen/X86/unfold-masked-merge-scalar-variablemask.ll


Index: llvm/test/CodeGen/X86/unfold-masked-merge-scalar-variablemask.ll
===================================================================
--- llvm/test/CodeGen/X86/unfold-masked-merge-scalar-variablemask.ll
+++ llvm/test/CodeGen/X86/unfold-masked-merge-scalar-variablemask.ll
@@ -705,10 +705,9 @@
 ;
 ; CHECK-BMI-LABEL: in_constant_varx_42_invmask:
 ; CHECK-BMI:       # %bb.0:
-; CHECK-BMI-NEXT:    notl %edx
-; CHECK-BMI-NEXT:    andnl %edx, %edi, %eax
-; CHECK-BMI-NEXT:    orl $42, %edx
-; CHECK-BMI-NEXT:    andnl %edx, %eax, %eax
+; CHECK-BMI-NEXT:    andnl %edi, %edx, %eax
+; CHECK-BMI-NEXT:    andl $42, %edx
+; CHECK-BMI-NEXT:    orl %edx, %eax
 ; CHECK-BMI-NEXT:    retq
   %notmask = xor i32 %mask, -1
   %n0 = xor i32 %x, 42 ; %x
Index: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7778,8 +7778,10 @@
 
   SDLoc DL(N);
 
-  // If Y is a constant, check that 'andn' works with immediates.
-  if (!TLI.hasAndNot(Y)) {
+  // If Y is a constant, check that 'andn' works with immediates. Unless M is
+  // a not that would already allow ANDN to be used.
+  // FIXME: If M is a not, do we need to check if X is a constant?
+  if (!TLI.hasAndNot(Y) && !isBitwiseNot(M)) {
     assert(TLI.hasAndNot(X) && "Only mask is a variable? Unreachable.");
     // If not, we need to do a bit more work to make sure andn is still used.
     SDValue NotX = DAG.getNOT(DL, X, VT);


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