[PATCH] D31119: [InstCombine] Teach SimplifyDemandedUseBits to shrink Constants on the left side of subtracts

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.

Subtracts can have constants on the left side, but we don't shrink them based on demanded bits. This patch fixes that to match the right hand side.


https://reviews.llvm.org/D31119

Files:
  lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
  test/Transforms/InstCombine/sub.ll


Index: test/Transforms/InstCombine/sub.ll
===================================================================
--- test/Transforms/InstCombine/sub.ll
+++ test/Transforms/InstCombine/sub.ll
@@ -701,3 +701,46 @@
   ret i8 %sub
 }
 
+define i32 @test49(i32 %X) {
+; CHECK-LABEL: @test49(
+; CHECK-NEXT:    [[SUB:%.*]] = sub i32 1, [[X:%.*]]
+; CHECK-NEXT:    [[RES:%.*]] = and i32 [[SUB]], 64
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+  %sub = sub i32 129, %X
+  %res = and i32 %sub, 64
+  ret i32 %res
+}
+
+define i32 @test50(i32 %X) {
+; CHECK-LABEL: @test50(
+; CHECK-NEXT:    [[SUB:%.*]] = sub i32 1, [[X:%.*]]
+; CHECK-NEXT:    [[RES:%.*]] = and i32 [[SUB]], 127
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+  %sub = sub i32 129, %X
+  %res = and i32 %sub, 127
+  ret i32 %res
+}
+
+define i32 @test51(i32 %X) {
+; CHECK-LABEL: @test51(
+; CHECK-NEXT:    [[SUB:%.*]] = sub i32 126, [[X:%.*]]
+; CHECK-NEXT:    [[RES:%.*]] = and i32 [[SUB]], 64
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+  %sub = sub i32 254, %X
+  %res = and i32 %sub, 64
+  ret i32 %res
+}
+
+define i32 @test52(i32 %X) {
+; CHECK-LABEL: @test52(
+; CHECK-NEXT:    [[SUB:%.*]] = sub i32 126, [[X:%.*]]
+; CHECK-NEXT:    [[RES:%.*]] = and i32 [[SUB]], 127
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+  %sub = sub i32 254, %X
+  %res = and i32 %sub, 127
+  ret i32 %res
+}
Index: lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -533,7 +533,8 @@
       // Right fill the mask of bits for this ADD/SUB to demand the most
       // significant bit and all those below it.
       APInt DemandedFromOps(APInt::getLowBitsSet(BitWidth, BitWidth-NLZ));
-      if (SimplifyDemandedBits(I->getOperandUse(0), DemandedFromOps,
+      if (ShrinkDemandedConstant(I, 0, DemandedFromOps) ||
+          SimplifyDemandedBits(I->getOperandUse(0), DemandedFromOps,
                                LHSKnownZero, LHSKnownOne, Depth + 1) ||
           ShrinkDemandedConstant(I, 1, DemandedFromOps) ||
           SimplifyDemandedBits(I->getOperandUse(1), DemandedFromOps,


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