InstCombine: Optimize -x s< cst

[majnemer]

Summary

This gets rid of a sub instruction by moving the negation to the
constant when valid.

[majnemer]

Closed by commit rL208827 (authored by @majnemer).

lib/Transforms/InstCombine/InstCombineCompares.cpp

if (B && D && B == D && NoOp0WrapProblem && NoOp1WrapProblem &&
       return new ICmpInst(Pred, A, C);
 
     // icmp (X-Y), (X-Z) -> icmp Z, Y for equalities or if there is no overflow.
     if (A && C && A == C && NoOp0WrapProblem && NoOp1WrapProblem &&
         // Try not to increase register pressure.
         BO0->hasOneUse() && BO1->hasOneUse())
       return new ICmpInst(Pred, D, B);
 
+    // icmp (0-X) < cst --> -cst < x
+    if (NoOp0WrapProblem && ICmpInst::isSigned(Pred)) {
+      Value *X;
+      if (match(BO0, m_Neg(m_Value(X))))
+        if (ConstantInt *RHSC = dyn_cast<ConstantInt>(Op1))
+          if (!RHSC->isMinValue(/*isSigned=*/true))
+            return new ICmpInst(I.getPredicate(), ConstantExpr::getNeg(RHSC), X);
+    }
+
     BinaryOperator *SRem = nullptr;
     // icmp (srem X, Y), Y
     if (BO0 && BO0->getOpcode() == Instruction::SRem &&
         Op1 == BO0->getOperand(1))
       SRem = BO0;
     // icmp Y, (srem X, Y)
     else if (BO1 && BO1->getOpcode() == Instruction::SRem &&
              Op0 == BO1->getOperand(1))

test/Transforms/InstCombine/icmp.ll

 ; CHECK: %z.unshifted = xor i32 %a, %b
 ; CHECK: %z = icmp ugt i32 %z.unshifted, 255
 define i1 @icmp_ashr_ashr_ne(i32 %a, i32 %b) nounwind {
  %x = ashr i32 %a, 8
  %y = ashr i32 %b, 8
  %z = icmp ne i32 %x, %y
  ret i1 %z
 }
+
+; CHECK-LABEL: @icmp_neg_cst_slt
+; CHECK-NEXT: [[CMP:%[a-z0-9]+]] = icmp sgt i32 %a, 10
+; CHECK-NEXT: ret i1 [[CMP]]
+define i1 @icmp_neg_cst_slt(i32 %a) {
+  %1 = sub nsw i32 0, %a
+  %2 = icmp slt i32 %1, -10
+  ret i1 %2
+}