[ValueTracking] Compute range for abs without nsw

This is a small followup to D59511. The code that was moved into
computeConstantRange() there is a bit overly conversative: If the
abs is not nsw, it does not compute any range. However, abs without
nsw still has a well-defined contiguous unsigned range from 0 to
SIGNED_MIN. This is a lot less useful than the usual 0 to SIGNED_MAX
range, but if we're already here we might as well specify it...

Differential Revision: https://reviews.llvm.org/D59563

llvm-svn: 356586

2 files changed, 9 insertions, 12 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 99d6010acee..52328b174b4 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -5673,14 +5673,15 @@ static void setLimitsForSelectPattern(const SelectInst &SI, APInt &Lower,
 
   unsigned BitWidth = SI.getType()->getScalarSizeInBits();
 
-  // matchSelectPattern() returns the negation part of an abs pattern in RHS.
-  // If the negate has an NSW flag, abs(INT_MIN) is undefined. Without that
-  // constraint, we can't make a contiguous range for the result of abs.
-  if (R.Flavor == SelectPatternFlavor::SPF_ABS &&
-      cast<Instruction>(RHS)->hasNoSignedWrap()) {
-    // The result of abs(X) is >= 0 (with nsw).
+  if (R.Flavor == SelectPatternFlavor::SPF_ABS) {
+    // If the negation part of the abs (in RHS) has the NSW flag,
+    // then the result of abs(X) is [0..SIGNED_MAX],
+    // otherwise it is [0..SIGNED_MIN], as -SIGNED_MIN == SIGNED_MIN.
     Lower = APInt::getNullValue(BitWidth);
-    Upper = APInt::getSignedMaxValue(BitWidth) + 1;
+    if (cast<Instruction>(RHS)->hasNoSignedWrap())
+      Upper = APInt::getSignedMaxValue(BitWidth) + 1;
+    else
+      Upper = APInt::getSignedMinValue(BitWidth) + 1;
     return;
   }
 
diff --git a/llvm/test/Transforms/InstSimplify/icmp-abs-nabs.ll b/llvm/test/Transforms/InstSimplify/icmp-abs-nabs.ll
index 52545e7de0b..be2e7b4d4a6 100644
--- a/llvm/test/Transforms/InstSimplify/icmp-abs-nabs.ll
+++ b/llvm/test/Transforms/InstSimplify/icmp-abs-nabs.ll
@@ -150,11 +150,7 @@ define i1 @abs_nsw_is_not_negative_wrong_range(i32 %x) {
 ; Even if we don't have nsw, the range is still limited in the unsigned domain.
 define i1 @abs_positive_or_signed_min(i32 %x) {
 ; CHECK-LABEL: @abs_positive_or_signed_min(
-; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
-; CHECK-NEXT:    [[NEGX:%.*]] = sub i32 0, [[X]]
-; CHECK-NEXT:    [[ABS:%.*]] = select i1 [[CMP]], i32 [[NEGX]], i32 [[X]]
-; CHECK-NEXT:    [[R:%.*]] = icmp ult i32 [[ABS]], -2147483647
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 true
 ;
   %cmp = icmp slt i32 %x, 0
   %negx = sub i32 0, %x