[PATCH] InstSimplify: Optimize signed icmp of -(zext V)

[David Majnemer]

Hi nicholas,

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

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

http://reviews.llvm.org/D3754

Files:
  lib/Analysis/InstructionSimplify.cpp
  test/Transforms/InstSimplify/compare.ll

Index: lib/Analysis/InstructionSimplify.cpp
===================================================================
--- lib/Analysis/InstructionSimplify.cpp
+++ lib/Analysis/InstructionSimplify.cpp
@@ -2280,6 +2280,24 @@
     }
   }
 
+  // 0 - (zext X) pred C
+  if (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 (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;
Index: test/Transforms/InstSimplify/compare.ll
===================================================================
--- test/Transforms/InstSimplify/compare.ll
+++ test/Transforms/InstSimplify/compare.ll
@@ -757,3 +757,43 @@
 ; 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
+; CHECK-NEXT: ret i1 false
+}
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