[ValueTracking] An improvement to IR ValueTracking on Non-negative Integers

Patch by Li Huang

Differential Revision: https://reviews.llvm.org/D18777

llvm-svn: 278267

3 files changed, 135 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 59c39e5..c4403c6 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1272,7 +1272,9 @@ static void computeKnownBitsFromOperator(Operator *I, APInt &KnownZero,
         unsigned Opcode = LU->getOpcode();
         // Check for operations that have the property that if
         // both their operands have low zero bits, the result
-        // will have low zero bits.
+        // will have low zero bits. Also check for operations 
+        // that are known to produce non-negative or negative
+        // recurrence values. 
         if (Opcode == Instruction::Add ||
             Opcode == Instruction::Sub ||
             Opcode == Instruction::And ||
@@ -1298,6 +1300,40 @@ static void computeKnownBitsFromOperator(Operator *I, APInt &KnownZero,
           KnownZero = APInt::getLowBitsSet(BitWidth,
                                            std::min(KnownZero2.countTrailingOnes(),
                                                     KnownZero3.countTrailingOnes()));
+
+          auto *OverflowOp = dyn_cast<OverflowingBinaryOperator>(LU);
+          if (OverflowOp && OverflowOp->hasNoSignedWrap()) {
+            // If initial value of recurrence is nonnegative, and we are adding 
+            // a nonnegative number with nsw, the result can only be nonnegative
+            // or poison value regardless of the number of times we execute the 
+            // add in phi recurrence. If initial value is negative and we are 
+            // adding a negative number with nsw, the result can only be 
+            // negative or poison value. Similar arguments apply to sub and mul.
+            //
+            // (add non-negative, non-negative) --> non-negative
+            // (add negative, negative) --> negative
+            if (Opcode == Instruction::Add) {
+              if (KnownZero2.isNegative() && KnownZero3.isNegative())
+                KnownZero.setBit(BitWidth - 1);
+              else if (KnownOne2.isNegative() && KnownOne3.isNegative())
+                KnownOne.setBit(BitWidth - 1);
+            }
+            
+            // (sub nsw non-negative, negative) --> non-negative
+            // (sub nsw negative, non-negative) --> negative
+            else if (Opcode == Instruction::Sub && LL == I) {
+              if (KnownZero2.isNegative() && KnownOne3.isNegative())
+                KnownZero.setBit(BitWidth - 1);
+              else if (KnownOne2.isNegative() && KnownZero3.isNegative())
+                KnownOne.setBit(BitWidth - 1);
+            }
+            
+            // (mul nsw non-negative, non-negative) --> non-negative
+            else if (Opcode == Instruction::Mul && KnownZero2.isNegative() && 
+                     KnownZero3.isNegative())
+              KnownZero.setBit(BitWidth - 1);
+          }
+
           break;
         }
       }
diff --git a/llvm/test/Analysis/ValueTracking/iv-known-sign.ll b/llvm/test/Analysis/ValueTracking/iv-known-sign.ll
new file mode 100644
index 0000000..303031b
--- /dev/null
+++ b/llvm/test/Analysis/ValueTracking/iv-known-sign.ll
@@ -0,0 +1,97 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+; Induction variable is known to be non-negative
+; when its initial value is non-negative and 
+; increments by non-negative value
+define i32 @test_indvar_nonnegative_add() {
+; CHECK-LABEL: @test_indvar_nonnegative_add(
+; CHECK: br i1 true, label %for.end, label %for.body
+entry:
+  br label %for.body
+
+for.body:
+  %i = phi i32 [0, %entry], [%inc, %for.body]
+  %inc = add nsw i32 %i, 1
+  %cmp = icmp sge i32 %i, 0
+  br i1 %cmp, label %for.end, label %for.body
+
+for.end:
+  ret i32 %i
+}
+
+; Induction variable is known to be non-negative
+; when its initial value is non-negative and 
+; is multiplied by a non-negative value in each 
+; iteration
+define i32 @test_indvar_nonnegative_mul() {
+; CHECK-LABEL: @test_indvar_nonnegative_mul(
+; CHECK: br i1 true, label %for.end, label %for.body
+entry:
+  br label %for.body
+
+for.body:
+  %i = phi i32 [1, %entry], [%inc, %for.body]
+  %inc = mul nsw i32 %i, 3
+  %cmp = icmp sge i32 %i, 0
+  br i1 %cmp, label %for.end, label %for.body
+
+for.end:
+  ret i32 %i
+}
+
+; Induction variable is known to be non-negative,
+; Similar to add
+define i32 @test_indvar_nonnegative_sub(i32 %a) {
+; CHECK-LABEL: @test_indvar_nonnegative_sub(
+; CHECK: br i1 true, label %for.end, label %for.body
+entry:
+  br label %for.body
+
+for.body:
+  %i = phi i32 [0, %entry], [%inc, %for.body]
+  %b = or i32 %a, -2147483648
+  %inc = sub nsw i32 %i, %b
+  %cmp = icmp sge i32 %i, 0
+  br i1 %cmp, label %for.end, label %for.body
+
+for.end:
+  ret i32 %i
+}
+
+; Induction variable is known to be negative when 
+; its initial value is negative and decrements by
+; a non-negative value
+define i32 @test_indvar_negative_add() {
+; CHECK-LABEL: @test_indvar_negative_add(
+; CHECK: br i1 true, label %for.end, label %for.body
+entry:
+  br label %for.body
+
+for.body:
+  %i = phi i32 [-1, %entry], [%inc, %for.body]
+  %inc = add nsw i32 %i, -1
+  %cmp = icmp slt i32 %i, 0
+  br i1 %cmp, label %for.end, label %for.body
+
+for.end:
+  ret i32 %i
+}
+
+; Induction variable is known to be negative,
+; similar to add
+define i32 @test_indvar_negative_sub(i32 %a) {
+; CHECK-LABEL: @test_indvar_negative_sub(
+; CHECK: br i1 true, label %for.end, label %for.body
+entry:
+  br label %for.body
+
+for.body:
+  %i = phi i32 [-1, %entry], [%inc, %for.body]
+  %b = and i32 %a, 2147483647
+  %inc = sub nsw i32 %i, %b
+  %cmp = icmp slt i32 %i, 0
+  br i1 %cmp, label %for.end, label %for.body
+
+for.end:
+  ret i32 %i
+}
diff --git a/llvm/test/Transforms/BBVectorize/loop1.ll b/llvm/test/Transforms/BBVectorize/loop1.ll
index 70a5def..445dec1 100644
--- a/llvm/test/Transforms/BBVectorize/loop1.ll
+++ b/llvm/test/Transforms/BBVectorize/loop1.ll
@@ -83,7 +83,7 @@ for.body:                                         ; preds = %for.body, %entry
 ; CHECK-UNRL: %add12 = fadd <2 x double> %add7, %mul11
 ; CHECK-UNRL: %4 = bitcast double* %arrayidx14 to <2 x double>*
 ; CHECK-UNRL: store <2 x double> %add12, <2 x double>* %4, align 8
-; CHECK-UNRL: %indvars.iv.next.1 = add nsw i64 %indvars.iv, 2
+; CHECK-UNRL: %indvars.iv.next.1 = add nuw nsw i64 %indvars.iv, 2
 ; CHECK-UNRL: %lftr.wideiv.1 = trunc i64 %indvars.iv.next.1 to i32
 ; CHECK-UNRL: %exitcond.1 = icmp eq i32 %lftr.wideiv.1, 10
 ; CHECK-UNRL: br i1 %exitcond.1, label %for.end, label %for.body