InstSimplify: Optimize signed icmp of -(zext V)

Summary:
We know that -(zext V) will always be <= zero, simplify signed icmps
that have these.

Uncovered using http://www.cs.utah.edu/~regehr/souper/

Reviewers: nicholas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3754

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208809 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index 4c621b0e9110492b7a01cba4b6795aad788ee57a..5421a3992267882da2bdeb40afa5fb060027b3be 100644 (file)
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/lib/Analysis/InstructionSimplify.cpp
@@ -2280,6 +2280,28 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
     }
   }
 
+  // 0 - (zext X) pred C
+  if (!CmpInst::isUnsigned(Pred) && match(LHS, m_Neg(m_ZExt(m_Value())))) {
+    if (ConstantInt *RHSC = dyn_cast<ConstantInt>(RHS)) {
+      if (RHSC->getValue().isStrictlyPositive()) {
+        if (Pred == ICmpInst::ICMP_SLT)
+          return ConstantInt::getTrue(RHSC->getContext());
+        if (Pred == ICmpInst::ICMP_SGE)
+          return ConstantInt::getFalse(RHSC->getContext());
+        if (Pred == ICmpInst::ICMP_EQ)
+          return ConstantInt::getFalse(RHSC->getContext());
+        if (Pred == ICmpInst::ICMP_NE)
+          return ConstantInt::getTrue(RHSC->getContext());
+      }
+      if (RHSC->getValue().isNonNegative()) {
+        if (Pred == ICmpInst::ICMP_SLE)
+          return ConstantInt::getTrue(RHSC->getContext());
+        if (Pred == ICmpInst::ICMP_SGT)
+          return ConstantInt::getFalse(RHSC->getContext());
+      }
+    }
+  }
+
   // icmp pred (urem X, Y), Y
   if (LBO && match(LBO, m_URem(m_Value(), m_Specific(RHS)))) {
     bool KnownNonNegative, KnownNegative;

diff --git a/test/Transforms/InstSimplify/compare.ll b/test/Transforms/InstSimplify/compare.ll
index ee6be0458cb3b8ba0b8daf7956ff132869003d86..76cff902c96f4726caf8a6a774d074d1c4395010 100644 (file)
--- a/test/Transforms/InstSimplify/compare.ll
+++ b/test/Transforms/InstSimplify/compare.ll
@@ -757,3 +757,63 @@ define <4 x i8> @vectorselectfold2(<4 x i8> %a, <4 x i8> %b) {
 ; CHECK-LABEL: @vectorselectfold
 ; CHECK-NEXT: ret <4 x i8> %a
 }
+
+define i1 @compare_always_true_slt(i16 %a) {
+  %1 = zext i16 %a to i32
+  %2 = sub nsw i32 0, %1
+  %3 = icmp slt i32 %2, 1
+  ret i1 %3
+
+; CHECK-LABEL: @compare_always_true_slt
+; CHECK-NEXT: ret i1 true
+}
+
+define i1 @compare_always_true_sle(i16 %a) {
+  %1 = zext i16 %a to i32
+  %2 = sub nsw i32 0, %1
+  %3 = icmp sle i32 %2, 0
+  ret i1 %3
+
+; CHECK-LABEL: @compare_always_true_sle
+; CHECK-NEXT: ret i1 true
+}
+
+define i1 @compare_always_false_sgt(i16 %a) {
+  %1 = zext i16 %a to i32
+  %2 = sub nsw i32 0, %1
+  %3 = icmp sgt i32 %2, 0
+  ret i1 %3
+
+; CHECK-LABEL: @compare_always_false_sgt
+; CHECK-NEXT: ret i1 false
+}
+
+define i1 @compare_always_false_sge(i16 %a) {
+  %1 = zext i16 %a to i32
+  %2 = sub nsw i32 0, %1
+  %3 = icmp sge i32 %2, 1
+  ret i1 %3
+
+; CHECK-LABEL: @compare_always_false_sge
+; CHECK-NEXT: ret i1 false
+}
+
+define i1 @compare_always_false_eq(i16 %a) {
+  %1 = zext i16 %a to i32
+  %2 = sub nsw i32 0, %1
+  %3 = icmp eq i32 %2, 1
+  ret i1 %3
+
+; CHECK-LABEL: @compare_always_false_eq
+; CHECK-NEXT: ret i1 false
+}
+
+define i1 @compare_always_false_ne(i16 %a) {
+  %1 = zext i16 %a to i32
+  %2 = sub nsw i32 0, %1
+  %3 = icmp ne i32 %2, 1
+  ret i1 %3
+
+; CHECK-LABEL: @compare_always_false_ne
+; CHECK-NEXT: ret i1 true
+}