[PATCH] D84971: [ValueTracking] Add ComputeNumSignBits support for llvm.abs intrinsic

[Craig Topper via Phabricator via llvm-commits]

This revision was automatically updated to reflect the committed changes.
Closed by commit rG0e0aebc5273c: [ValueTracking] Add ComputeNumSignBits support for llvm.abs intrinsic (authored by craig.topper).

Changed prior to commit:
  https://reviews.llvm.org/D84971?vs=282014&id=282270#toc

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D84971/new/

https://reviews.llvm.org/D84971

Files:
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/test/Transforms/InstCombine/abs-intrinsic.ll


Index: llvm/test/Transforms/InstCombine/abs-intrinsic.ll
===================================================================
--- llvm/test/Transforms/InstCombine/abs-intrinsic.ll
+++ llvm/test/Transforms/InstCombine/abs-intrinsic.ll
@@ -51,3 +51,18 @@
   %and2 = and i32 %abs, -4
   ret i32 %and2
 }
+
+; Make sure we infer this add doesn't overflow. The input to the abs has 3
+; sign bits, the abs reduces this to 2 sign bits.
+define i32 @abs_signbits(i30 %x) {
+; CHECK-LABEL: @abs_signbits(
+; CHECK-NEXT:    [[AND:%.*]] = sext i30 [[X:%.*]] to i32
+; CHECK-NEXT:    [[ABS:%.*]] = call i32 @llvm.abs.i32(i32 [[AND]], i1 false)
+; CHECK-NEXT:    [[ADD:%.*]] = add nsw i32 [[ABS]], 1
+; CHECK-NEXT:    ret i32 [[ADD]]
+;
+  %ext = sext i30 %x to i32
+  %abs = call i32 @llvm.abs.i32(i32 %ext, i1 false)
+  %add = add i32 %abs, 1
+  ret i32 %add
+}
Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -3001,6 +3001,19 @@
              "Failed to determine minimum sign bits");
       return Tmp;
     }
+    case Instruction::Call: {
+      if (const auto *II = dyn_cast<IntrinsicInst>(U)) {
+        switch (II->getIntrinsicID()) {
+        default: break;
+        case Intrinsic::abs:
+          Tmp = ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
+          if (Tmp == 1) break;
+
+          // Absolute value reduces number of sign bits by at most 1.
+          return Tmp - 1;
+        }
+      }
+    }
     }
   }
 


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