[PATCH] D47041: [ValueTracking] Teach computeKnownBits that the result of an absolute value pattern that uses nsw flag is always positive.

[Craig Topper via Phabricator via llvm-commits]

craig.topper updated this revision to Diff 148111.
craig.topper added a comment.

Add a combine to negate ABS/NABS to recover the broken tests.


https://reviews.llvm.org/D47041

Files:
  lib/Analysis/ValueTracking.cpp
  lib/Transforms/InstCombine/InstCombineAddSub.cpp
  test/Transforms/InstCombine/abs_abs.ll


Index: test/Transforms/InstCombine/abs_abs.ll
===================================================================
--- test/Transforms/InstCombine/abs_abs.ll
+++ test/Transforms/InstCombine/abs_abs.ll
@@ -933,8 +933,8 @@
 ; CHECK-LABEL: @nabs_abs_x09(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
 ; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 0, [[X]]
-; CHECK-NEXT:    [[COND1:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
-; CHECK-NEXT:    ret i32 [[COND1]]
+; CHECK-NEXT:    [[SUB16:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
+; CHECK-NEXT:    ret i32 [[SUB16]]
 ;
   %cmp = icmp sgt i32 %x, -1
   %sub = sub nsw i32 0, %x
@@ -949,8 +949,8 @@
 ; CHECK-LABEL: @nabs_abs_x10(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
 ; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 0, [[X]]
-; CHECK-NEXT:    [[COND1:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
-; CHECK-NEXT:    ret i32 [[COND1]]
+; CHECK-NEXT:    [[SUB16:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
+; CHECK-NEXT:    ret i32 [[SUB16]]
 ;
   %cmp = icmp sgt i32 %x, 0
   %sub = sub nsw i32 0, %x
@@ -965,8 +965,8 @@
 ; CHECK-LABEL: @nabs_abs_x11(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
 ; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 0, [[X]]
-; CHECK-NEXT:    [[COND1:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
-; CHECK-NEXT:    ret i32 [[COND1]]
+; CHECK-NEXT:    [[SUB16:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
+; CHECK-NEXT:    ret i32 [[SUB16]]
 ;
   %cmp = icmp slt i32 %x, 0
   %sub = sub nsw i32 0, %x
@@ -981,8 +981,8 @@
 ; CHECK-LABEL: @nabs_abs_x12(
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[X:%.*]], 0
 ; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 0, [[X]]
-; CHECK-NEXT:    [[COND1:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
-; CHECK-NEXT:    ret i32 [[COND1]]
+; CHECK-NEXT:    [[SUB16:%.*]] = select i1 [[CMP]], i32 [[X]], i32 [[SUB]]
+; CHECK-NEXT:    ret i32 [[SUB16]]
 ;
   %cmp = icmp slt i32 %x, 1
   %sub = sub nsw i32 0, %x
Index: lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1628,6 +1628,18 @@
         Value *ShAmtOp = cast<Instruction>(Op1)->getOperand(1);
         return BinaryOperator::CreateLShr(X, ShAmtOp);
       }
+
+      if (Op1->hasOneUse()) {
+        Value *LHS, *RHS;
+        SelectPatternFlavor SPF = matchSelectPattern(Op1, LHS, RHS).Flavor;
+        if (SPF == SPF_ABS || SPF == SPF_NABS) {
+          // This is a negate of an ABS/NABS pattern. Just swap the operands
+          // of the select.
+          SelectInst *SI = cast<SelectInst>(Op1);
+          return SelectInst::Create(SI->getCondition(), SI->getFalseValue(),
+                                    SI->getTrueValue());
+        }
+      }
     }
 
     // Turn this into a xor if LHS is 2^n-1 and the remaining bits are known
Index: lib/Analysis/ValueTracking.cpp
===================================================================
--- lib/Analysis/ValueTracking.cpp
+++ lib/Analysis/ValueTracking.cpp
@@ -1078,6 +1078,12 @@
       // leading zero bits.
       MaxHighZeros =
           std::max(Known.countMinLeadingZeros(), Known2.countMinLeadingZeros());
+    } else if (SPF == SPF_ABS) {
+      // RHS from matchSelectPattern returns the negation part of abs pattern.
+      // If the negate has an NSW flag we can assume the sign bit of the result
+      // will be 0 because that makes abs(INT_MIN) undefined.
+      if (cast<Instruction>(RHS)->hasNoSignedWrap())
+        MaxHighZeros = 1;
     }
 
     // Only known if known in both the LHS and RHS.


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