[PATCH] D22477: [InstCombine] fold add(zext(xor X, C), C) --> sext X when C is INT_MIN in the source type

[Sanjay Patel via llvm-commits]

spatel created this revision.
spatel added reviewers: majnemer, eli.friedman, sanjoy.
spatel added a subscriber: llvm-commits.
Herald added a subscriber: mcrosier.

The pattern may look more obviously like a sext if written as:
  define i32 @g(i16 %x) {
    %zext = zext i16 %x to i32
    %xor = xor i32 %zext, 32768
    %add = add i32 %xor, -32768
    ret i32 %add
  }

We already have that fold in visitAdd().

And that probably gives away how I got here: I was testing if we had any missing folds if we make D22271 more liberal about pulling logic ops ahead of zexts.

https://reviews.llvm.org/D22477

Files:
  lib/Transforms/InstCombine/InstCombineAddSub.cpp
  test/Transforms/InstCombine/apint-add.ll

Index: test/Transforms/InstCombine/apint-add.ll
===================================================================
--- test/Transforms/InstCombine/apint-add.ll
+++ test/Transforms/InstCombine/apint-add.ll
@@ -56,9 +56,7 @@
 
 define i7 @sext(i4 %x) {
 ; CHECK-LABEL: @sext(
-; CHECK-NEXT:    [[XOR:%.*]] = xor i4 %x, -8
-; CHECK-NEXT:    [[ZEXT:%.*]] = zext i4 [[XOR]] to i7
-; CHECK-NEXT:    [[ADD:%.*]] = add nsw i7 [[ZEXT]], -8
+; CHECK-NEXT:    [[ADD:%.*]] = sext i4 %x to i7
 ; CHECK-NEXT:    ret i7 [[ADD]]
 ;
   %xor = xor i4 %x, -8
@@ -69,9 +67,7 @@
 
 define <2 x i10> @sext_vec(<2 x i3> %x) {
 ; CHECK-LABEL: @sext_vec(
-; CHECK-NEXT:    [[XOR:%.*]] = xor <2 x i3> %x, <i3 -4, i3 -4>
-; CHECK-NEXT:    [[ZEXT:%.*]] = zext <2 x i3> [[XOR]] to <2 x i10>
-; CHECK-NEXT:    [[ADD:%.*]] = add nsw <2 x i10> [[ZEXT]], <i10 -4, i10 -4>
+; CHECK-NEXT:    [[ADD:%.*]] = sext <2 x i3> %x to <2 x i10>
 ; CHECK-NEXT:    ret <2 x i10> [[ADD]]
 ;
   %xor = xor <2 x i3> %x, <i3 -4, i3 -4>
Index: lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1047,6 +1047,16 @@
     // X + (signbit) --> X ^ signbit
     if (Val->isSignBit())
       return BinaryOperator::CreateXor(LHS, RHS);
+
+    // Is this add the last step in a convoluted sext?
+    Value *X;
+    const APInt *C;
+    if (match(LHS, m_OneUse(m_ZExt(m_OneUse(m_Xor(m_Value(X), m_APInt(C)))))) &&
+        C->isMinSignedValue() &&
+        C->sext(LHS->getType()->getScalarSizeInBits()) == *Val) {
+      // add(zext(xor i16 X, -32768), -32768) --> sext X
+      return CastInst::Create(Instruction::SExt, X, LHS->getType());
+    }
   }
 
   // FIXME: Use the match above instead of dyn_cast to allow these transforms


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