[InstCombine] Don't transform sub X, ~Y -> add X, -Y unless Y is actually negatable

[goldsteinn]

• [InstCombine] Add tests for improving sub X, ~Y -> add X, -Y; NFC
• [InstCombine] Don't transform sub X, ~Y -> add X, -Y unless Y is actually negatable

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: None (goldsteinn)

Changes

• [InstCombine] Add tests for improving sub X, ~Y -> add X, -Y; NFC
• [InstCombine] Don't transform sub X, ~Y -> add X, -Y unless Y is actually negatable

---

Full diff: https://github.com/llvm/llvm-project/pull/72767.diff

3 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp (+5-3)
• (modified) llvm/test/Analysis/ValueTracking/knownbits-and-or-xor-lowbit.ll (+2-3)
• (modified) llvm/test/Transforms/InstCombine/fold-sub-of-not-to-inc-of-add.ll (+20-7)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp b/llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp
index 53b34b03ca78070..6e0b44c11f22a45 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp
@@ -447,9 +447,11 @@ std::array<Value *, 2> Negator::getSortedOperandsOfBinOp(Instruction *I) {
     // `xor` is negatible if one of its operands is invertible.
     // FIXME: InstCombineInverter? But how to connect Inverter and Negator?
     if (auto *C = dyn_cast<Constant>(Ops[1])) {
-      Value *Xor = Builder.CreateXor(Ops[0], ConstantExpr::getNot(C));
-      return Builder.CreateAdd(Xor, ConstantInt::get(Xor->getType(), 1),
-                               I->getName() + ".neg");
+      if (IsTrulyNegation) {
+        Value *Xor = Builder.CreateXor(Ops[0], ConstantExpr::getNot(C));
+        return Builder.CreateAdd(Xor, ConstantInt::get(Xor->getType(), 1),
+                                 I->getName() + ".neg");
+      }
     }
     return nullptr;
   }
diff --git a/llvm/test/Analysis/ValueTracking/knownbits-and-or-xor-lowbit.ll b/llvm/test/Analysis/ValueTracking/knownbits-and-or-xor-lowbit.ll
index 7fbc78079c24029..f44948e6a089f88 100644
--- a/llvm/test/Analysis/ValueTracking/knownbits-and-or-xor-lowbit.ll
+++ b/llvm/test/Analysis/ValueTracking/knownbits-and-or-xor-lowbit.ll
@@ -106,9 +106,8 @@ define <2 x i1> @sub_XY_and_bit0_is_zero_fail(<2 x i8> %x, <2 x i8> %C) nounwind
 
 define i1 @sub_XY_xor_bit0_is_one_fail(i8 %x, i8 %C) nounwind {
 ; CHECK-LABEL: @sub_XY_xor_bit0_is_one_fail(
-; CHECK-NEXT:    [[TMP1:%.*]] = xor i8 [[C:%.*]], -2
-; CHECK-NEXT:    [[C1_NEG:%.*]] = add i8 [[TMP1]], 1
-; CHECK-NEXT:    [[Y:%.*]] = add i8 [[C1_NEG]], [[X:%.*]]
+; CHECK-NEXT:    [[C1:%.*]] = xor i8 [[C:%.*]], 1
+; CHECK-NEXT:    [[Y:%.*]] = sub i8 [[X:%.*]], [[C1]]
 ; CHECK-NEXT:    [[W:%.*]] = xor i8 [[Y]], [[X]]
 ; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[W]], 10
 ; CHECK-NEXT:    ret i1 [[R]]
diff --git a/llvm/test/Transforms/InstCombine/fold-sub-of-not-to-inc-of-add.ll b/llvm/test/Transforms/InstCombine/fold-sub-of-not-to-inc-of-add.ll
index 1e315696c3110c3..6f311f05fb0176e 100644
--- a/llvm/test/Transforms/InstCombine/fold-sub-of-not-to-inc-of-add.ll
+++ b/llvm/test/Transforms/InstCombine/fold-sub-of-not-to-inc-of-add.ll
@@ -22,6 +22,19 @@ define i32 @p0_scalar(i32 %x, i32 %y) {
   ret i32 %t1
 }
 
+define i8 @p0_scalar_not_truly_negatable(i8 %x, i8 %y) {
+; CHECK-LABEL: @p0_scalar_not_truly_negatable(
+; CHECK-NEXT:    [[XX:%.*]] = xor i8 [[X:%.*]], 123
+; CHECK-NEXT:    [[YY:%.*]] = xor i8 [[Y:%.*]], 45
+; CHECK-NEXT:    [[R:%.*]] = sub i8 [[XX]], [[YY]]
+; CHECK-NEXT:    ret i8 [[R]]
+;
+  %xx = xor i8 %x, 123
+  %yy = xor i8 %y, 45
+  %r = sub i8 %xx, %yy
+  ret i8 %r
+}
+
 ;------------------------------------------------------------------------------;
 ; Vector tests
 ;------------------------------------------------------------------------------;
@@ -79,28 +92,28 @@ define i32 @n4(i32 %x, i32 %y) {
 ; CHECK-NEXT:    ret i32 [[T1]]
 ;
   %t0 = xor i32 %x, -1
-  %t1 = sub i32 %t0, %y ; swapped
+  %t1 = sub i32 %t0, %y  ; swapped
   ret i32 %t1
 }
 
 define i32 @n5_is_not_not(i32 %x, i32 %y) {
 ; CHECK-LABEL: @n5_is_not_not(
-; CHECK-NEXT:    [[T0_NEG:%.*]] = add i32 [[X:%.*]], -2147483647
-; CHECK-NEXT:    [[T1:%.*]] = add i32 [[T0_NEG]], [[Y:%.*]]
+; CHECK-NEXT:    [[T0:%.*]] = xor i32 [[X:%.*]], 2147483647
+; CHECK-NEXT:    [[T1:%.*]] = sub i32 [[Y:%.*]], [[T0]]
 ; CHECK-NEXT:    ret i32 [[T1]]
 ;
-  %t0 = xor i32 %x, 2147483647 ; not -1
+  %t0 = xor i32 %x, 2147483647  ; not -1
   %t1 = sub i32 %y, %t0
   ret i32 %t1
 }
 
 define <2 x i32> @n5_is_not_not_vec_splat(<2 x i32> %x, <2 x i32> %y) {
 ; CHECK-LABEL: @n5_is_not_not_vec_splat(
-; CHECK-NEXT:    [[T0_NEG:%.*]] = add <2 x i32> [[X:%.*]], <i32 -2147483647, i32 -2147483647>
-; CHECK-NEXT:    [[T1:%.*]] = add <2 x i32> [[T0_NEG]], [[Y:%.*]]
+; CHECK-NEXT:    [[T0:%.*]] = xor <2 x i32> [[X:%.*]], <i32 2147483647, i32 2147483647>
+; CHECK-NEXT:    [[T1:%.*]] = sub <2 x i32> [[Y:%.*]], [[T0]]
 ; CHECK-NEXT:    ret <2 x i32> [[T1]]
 ;
-  %t0 = xor <2 x i32> %x, <i32 2147483647, i32 2147483647> ; signmask, but not -1
+  %t0 = xor <2 x i32> %x, <i32 2147483647, i32 2147483647>  ; signmask, but not -1
   %t1 = sub <2 x i32> %y, %t0
   ret <2 x i32> %t1
 }

[dtcxzyw]

LGTM.

[nikic]

LGTM