[PATCH] D29729: [InstCombine] don't lose nsw/nuw from add by converting to xor

[Sanjay Patel via Phabricator via llvm-commits]

This revision was automatically updated to reflect the committed changes.
Closed by commit rL295574: [InstCombine] add nsw/nuw X, signbit --> or X, signbit (authored by spatel).

Changed prior to commit:
  https://reviews.llvm.org/D29729?vs=88197&id=89047#toc

Repository:
  rL LLVM

https://reviews.llvm.org/D29729

Files:
  llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
  llvm/trunk/test/Transforms/InstCombine/add.ll
  llvm/trunk/test/Transforms/InstCombine/icmp-add.ll


Index: llvm/trunk/test/Transforms/InstCombine/icmp-add.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/icmp-add.ll
+++ llvm/trunk/test/Transforms/InstCombine/icmp-add.ll
@@ -223,24 +223,22 @@
   ret i1 %c
 }
 
-; FIXME: InstCombine should not lose wrapping information by changing the add to xor.
+; InstCombine should not thwart this opportunity to simplify completely.
 
 define i1 @slt_zero_add_nsw_signbit(i8 %x) {
 ; CHECK-LABEL: @slt_zero_add_nsw_signbit(
-; CHECK-NEXT:    [[Z:%.*]] = icmp sgt i8 %x, -1
-; CHECK-NEXT:    ret i1 [[Z]]
+; CHECK-NEXT:    ret i1 true
 ;
   %y = add nsw i8 %x, -128
   %z = icmp slt i8 %y, 0
   ret i1 %z
 }
 
-; FIXME: InstCombine should not lose wrapping information by changing the add to xor.
+; InstCombine should not thwart this opportunity to simplify completely.
 
 define i1 @slt_zero_add_nuw_signbit(i8 %x) {
 ; CHECK-LABEL: @slt_zero_add_nuw_signbit(
-; CHECK-NEXT:    [[Z:%.*]] = icmp sgt i8 %x, -1
-; CHECK-NEXT:    ret i1 [[Z]]
+; CHECK-NEXT:    ret i1 true
 ;
   %y = add nuw i8 %x, 128
   %z = icmp slt i8 %y, 0
Index: llvm/trunk/test/Transforms/InstCombine/add.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/add.ll
+++ llvm/trunk/test/Transforms/InstCombine/add.ll
@@ -267,24 +267,22 @@
   ret i32 %add
 }
 
-; Lose no-wrap info by converting to xor? %x is known non-negative
-; here, but not after converting to xor.
+; No-wrap info allows converting the add to 'or'.
 
 define i8 @add_nsw_signbit(i8 %x) {
 ; CHECK-LABEL: @add_nsw_signbit(
-; CHECK-NEXT:    [[Y:%.*]] = xor i8 %x, -128
+; CHECK-NEXT:    [[Y:%.*]] = or i8 %x, -128
 ; CHECK-NEXT:    ret i8 [[Y]]
 ;
   %y = add nsw i8 %x, -128
   ret i8 %y
 }
 
-; Lose no-wrap info by converting to xor? %x is known non-negative
-; (x < 128 unsigned), but not after converting to xor.
+; No-wrap info allows converting the add to 'or'.
 
 define i8 @add_nuw_signbit(i8 %x) {
 ; CHECK-LABEL: @add_nuw_signbit(
-; CHECK-NEXT:    [[Y:%.*]] = xor i8 %x, -128
+; CHECK-NEXT:    [[Y:%.*]] = or i8 %x, -128
 ; CHECK-NEXT:    ret i8 [[Y]]
 ;
   %y = add nuw i8 %x, 128
Index: llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1044,9 +1044,16 @@
 
   const APInt *Val;
   if (match(RHS, m_APInt(Val))) {
-    // X + (signbit) --> X ^ signbit
-    if (Val->isSignBit())
+    if (Val->isSignBit()) {
+      // If wrapping is not allowed, then the addition must set the sign bit:
+      // X + (signbit) --> X | signbit
+      if (I.hasNoSignedWrap() || I.hasNoUnsignedWrap())
+        return BinaryOperator::CreateOr(LHS, RHS);
+
+      // If wrapping is allowed, then the addition flips the sign bit of LHS:
+      // X + (signbit) --> X ^ signbit
       return BinaryOperator::CreateXor(LHS, RHS);
+    }
 
     // Is this add the last step in a convoluted sext?
     Value *X;


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