[ValueTracking] Teach computeKnownBits about [su]min/max

Reasoning about a select in terms of a min or max allows us to derive a
tigher bound on the result.

llvm-svn: 277914

4 files changed, 69 insertions, 8 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index f2b40787443..c3a5e6af62b 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -950,14 +950,63 @@ static void computeKnownBitsFromOperator(Operator *I, APInt &KnownZero,
     KnownZero = APInt::getHighBitsSet(BitWidth, LeadZ);
     break;
   }
-  case Instruction::Select:
+  case Instruction::Select: {
     computeKnownBits(I->getOperand(2), KnownZero, KnownOne, Depth + 1, Q);
     computeKnownBits(I->getOperand(1), KnownZero2, KnownOne2, Depth + 1, Q);
 
+    Value *LHS, *RHS;
+    SelectPatternFlavor SPF = matchSelectPattern(I, LHS, RHS).Flavor;
+    if (SelectPatternResult::isMinOrMax(SPF)) {
+      computeKnownBits(RHS, KnownZero, KnownOne, Depth + 1, Q);
+      computeKnownBits(LHS, KnownZero2, KnownOne2, Depth + 1, Q);
+    } else {
+      computeKnownBits(I->getOperand(2), KnownZero, KnownOne, Depth + 1, Q);
+      computeKnownBits(I->getOperand(1), KnownZero2, KnownOne2, Depth + 1, Q);
+    }
+
+    unsigned MaxHighOnes = 0;
+    unsigned MaxHighZeros = 0;
+    if (SPF == SPF_SMAX) {
+      // If both sides are negative, the result is negative.
+      if (KnownOne[BitWidth - 1] && KnownOne2[BitWidth - 1])
+        // We can derive a lower bound on the result by taking the max of the
+        // leading one bits.
+        MaxHighOnes =
+            std::max(KnownOne.countLeadingOnes(), KnownOne2.countLeadingOnes());
+      // If either side is non-negative, the result is non-negative.
+      else if (KnownZero[BitWidth - 1] || KnownZero2[BitWidth - 1])
+        MaxHighZeros = 1;
+    } else if (SPF == SPF_SMIN) {
+      // If both sides are non-negative, the result is non-negative.
+      if (KnownZero[BitWidth - 1] && KnownZero2[BitWidth - 1])
+        // We can derive an upper bound on the result by taking the max of the
+        // leading zero bits.
+        MaxHighZeros = std::max(KnownZero.countLeadingOnes(),
+                                KnownZero2.countLeadingOnes());
+      // If either side is negative, the result is negative.
+      else if (KnownOne[BitWidth - 1] || KnownOne2[BitWidth - 1])
+        MaxHighOnes = 1;
+    } else if (SPF == SPF_UMAX) {
+      // We can derive a lower bound on the result by taking the max of the
+      // leading one bits.
+      MaxHighOnes =
+          std::max(KnownOne.countLeadingOnes(), KnownOne2.countLeadingOnes());
+    } else if (SPF == SPF_UMIN) {
+      // We can derive an upper bound on the result by taking the max of the
+      // leading zero bits.
+      MaxHighZeros =
+          std::max(KnownZero.countLeadingOnes(), KnownZero2.countLeadingOnes());
+    }
+
     // Only known if known in both the LHS and RHS.
     KnownOne &= KnownOne2;
     KnownZero &= KnownZero2;
+    if (MaxHighOnes > 0)
+      KnownOne |= APInt::getHighBitsSet(BitWidth, MaxHighOnes);
+    if (MaxHighZeros > 0)
+      KnownZero |= APInt::getHighBitsSet(BitWidth, MaxHighZeros);
     break;
+  }
   case Instruction::FPTrunc:
   case Instruction::FPExt:
   case Instruction::FPToUI:
diff --git a/llvm/test/Transforms/InstCombine/sext.ll b/llvm/test/Transforms/InstCombine/sext.ll
index f04afcc747b..4caa0f1fbb1 100644
--- a/llvm/test/Transforms/InstCombine/sext.ll
+++ b/llvm/test/Transforms/InstCombine/sext.ll
@@ -193,3 +193,15 @@ define i32 @test17(i1 %x) nounwind {
 ; CHECK-NEXT: [[TEST17:%.*]] = zext i1 %x to i32
 ; CHECK-NEXT: ret i32 [[TEST17]]
 }
+
+define i32 @test18(i16 %x) {
+  %cmp = icmp slt i16 %x, 0
+  %sel = select i1 %cmp, i16 0, i16 %x
+  %ext = sext i16 %sel to i32
+  ret i32 %ext
+; CHECK-LABEL: @test18(
+; CHECK-NEXT: %[[cmp:.*]] = icmp slt i16 %x, 0
+; CHECK-NEXT: %[[sel:.*]] = select i1 %[[cmp]], i16 0, i16 %x
+; CHECK-NEXT: %[[ext:.*]] = zext i16 %[[sel]] to i32
+; CHECK-NEXT: ret i32 %[[ext]]
+}
diff --git a/llvm/test/Transforms/LoopVectorize/interleaved-accesses-pred-stores.ll b/llvm/test/Transforms/LoopVectorize/interleaved-accesses-pred-stores.ll
index 9ee6e6d529a..99a063b8c6e 100644
--- a/llvm/test/Transforms/LoopVectorize/interleaved-accesses-pred-stores.ll
+++ b/llvm/test/Transforms/LoopVectorize/interleaved-accesses-pred-stores.ll
@@ -13,7 +13,7 @@ target datalayout = "e-m:e-i64:64-i128:128-n32:64-S128"
 ; CHECK:   %n.mod.vf = and i64 %[[N:.+]], 1
 ; CHECK:   %[[IsZero:[a-zA-Z0-9]+]] = icmp eq i64 %n.mod.vf, 0
 ; CHECK:   %[[R:.+]] = select i1 %[[IsZero]], i64 2, i64 %n.mod.vf
-; CHECK:   %n.vec = sub i64 %[[N]], %[[R]]
+; CHECK:   %n.vec = sub nsw i64 %[[N]], %[[R]]
 ;
 ; CHECK: vector.body:
 ; CHECK:   %wide.vec = load <4 x i64>, <4 x i64>* %{{.*}}
@@ -62,7 +62,7 @@ for.end:
 ; CHECK:   %n.mod.vf = and i64 %[[N:.+]], 1
 ; CHECK:   %[[IsZero:[a-zA-Z0-9]+]] = icmp eq i64 %n.mod.vf, 0
 ; CHECK:   %[[R:.+]] = select i1 %[[IsZero]], i64 2, i64 %n.mod.vf
-; CHECK:   %n.vec = sub i64 %[[N]], %[[R]]
+; CHECK:   %n.vec = sub nsw i64 %[[N]], %[[R]]
 ;
 ; CHECK: vector.body:
 ; CHECK:   %[[L1:.+]] = load <4 x i64>, <4 x i64>* %{{.*}}
@@ -121,7 +121,7 @@ for.end:
 ; CHECK:   %n.mod.vf = and i64 %[[N:.+]], 1
 ; CHECK:   %[[IsZero:[a-zA-Z0-9]+]] = icmp eq i64 %n.mod.vf, 0
 ; CHECK:   %[[R:.+]] = select i1 %[[IsZero]], i64 2, i64 %n.mod.vf
-; CHECK:   %n.vec = sub i64 %[[N]], %[[R]]
+; CHECK:   %n.vec = sub nsw i64 %[[N]], %[[R]]
 ;
 ; CHECK: vector.body:
 ; CHECK:   %[[L1:.+]] = load <4 x i64>, <4 x i64>* %{{.*}}
diff --git a/llvm/test/Transforms/LoopVectorize/interleaved-accesses.ll b/llvm/test/Transforms/LoopVectorize/interleaved-accesses.ll
index 868c3a2cdab..34998782aa8 100644
--- a/llvm/test/Transforms/LoopVectorize/interleaved-accesses.ll
+++ b/llvm/test/Transforms/LoopVectorize/interleaved-accesses.ll
@@ -577,7 +577,7 @@ for.body:                                         ; preds = %for.body, %entry
 ; CHECK:   %n.mod.vf = and i64 %[[N:.+]], 3
 ; CHECK:   %[[IsZero:[a-zA-Z0-9]+]] = icmp eq i64 %n.mod.vf, 0
 ; CHECK:   %[[R:[a-zA-Z0-9]+]] = select i1 %[[IsZero]], i64 4, i64 %n.mod.vf
-; CHECK:   %n.vec = sub i64 %[[N]], %[[R]]
+; CHECK:   %n.vec = sub nsw i64 %[[N]], %[[R]]
 ; CHECK: vector.body:
 ; CHECK:   %[[L1:.+]] = load <8 x i32>, <8 x i32>* {{.*}}
 ; CHECK:   %[[X1:.+]] = extractelement <8 x i32> %[[L1]], i32 0
@@ -625,7 +625,7 @@ for.end:
 ; CHECK:   %n.mod.vf = and i64 %[[N:.+]], 3
 ; CHECK:   %[[IsZero:[a-zA-Z0-9]+]] = icmp eq i64 %n.mod.vf, 0
 ; CHECK:   %[[R:[a-zA-Z0-9]+]] = select i1 %[[IsZero]], i64 4, i64 %n.mod.vf
-; CHECK:   %n.vec = sub i64 %[[N]], %[[R]]
+; CHECK:   %n.vec = sub nsw i64 %[[N]], %[[R]]
 ; CHECK: vector.body:
 ; CHECK:   %[[Phi:.+]] = phi <4 x i32> [ zeroinitializer, %vector.ph ], [ {{.*}}, %vector.body ]
 ; CHECK:   %[[L1:.+]] = load <8 x i32>, <8 x i32>* {{.*}}
@@ -678,7 +678,7 @@ for.end:
 ; CHECK:   %n.mod.vf = and i64 %[[N:.+]], 3
 ; CHECK:   %[[IsZero:[a-zA-Z0-9]+]] = icmp eq i64 %n.mod.vf, 0
 ; CHECK:   %[[R:[a-zA-Z0-9]+]] = select i1 %[[IsZero]], i64 4, i64 %n.mod.vf
-; CHECK:   %n.vec = sub i64 %[[N]], %[[R]]
+; CHECK:   %n.vec = sub nsw i64 %[[N]], %[[R]]
 ; CHECK: vector.body:
 ; CHECK:   %[[L1:.+]] = load <8 x i32>, <8 x i32>* {{.*}}
 ; CHECK:   %[[X1:.+]] = extractelement <8 x i32> %[[L1]], i32 0
@@ -726,7 +726,7 @@ for.end:
 ; CHECK:   %n.mod.vf = and i64 %[[N:.+]], 3
 ; CHECK:   %[[IsZero:[a-zA-Z0-9]+]] = icmp eq i64 %n.mod.vf, 0
 ; CHECK:   %[[R:[a-zA-Z0-9]+]] = select i1 %[[IsZero]], i64 4, i64 %n.mod.vf
-; CHECK:   %n.vec = sub i64 %[[N]], %[[R]]
+; CHECK:   %n.vec = sub nsw i64 %[[N]], %[[R]]
 ; CHECK: vector.body:
 ; CHECK:   %[[Phi:.+]] = phi <4 x i32> [ zeroinitializer, %vector.ph ], [ {{.*}}, %vector.body ]
 ; CHECK:   %[[L1:.+]] = load <8 x i32>, <8 x i32>* {{.*}}