InstSimplify: Optimize signed icmp of -(zext V)

[majnemer]

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/

[majnemer]

Closed by commit rL208809 (authored by @majnemer).

lib/Analysis/InstructionSimplify.cpp

if (A && C && (A == C || A == D || B == C || B == D) &&
         Y = A;
         Z = C;
       }
       if (Value *V = SimplifyICmpInst(Pred, Y, Z, Q, MaxRecurse-1))
         return V;
     }
   }
 
+  // 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;
     switch (Pred) {
     default:
       break;
     case ICmpInst::ICMP_SGT:
     case ICmpInst::ICMP_SGE:

test/Transforms/InstSimplify/compare.ll

 define <4 x i8> @vectorselectfold2(<4 x i8> %a, <4 x i8> %b) {
   %true = icmp eq <4 x i8> zeroinitializer, zeroinitializer
   %sel = select <4 x i1> %true, <4 x i8> %a, <4 x i8> %b
   ret <4 x i8> %sel
 
 ; 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
+}