[llvm] r221045 - InstCombine: Combine (X+cst) < 0 --> X < -cst

[David Majnemer]

Author: majnemer
Date: Sat Nov  1 04:09:51 2014
New Revision: 221045

URL: http://llvm.org/viewvc/llvm-project?rev=221045&view=rev
Log:
InstCombine: Combine (X+cst) < 0 --> X < -cst

This can happen pretty often in code that looks like:
int foo = bar - 1;
if (foo < 0)
  do stuff

In this case, bar < 1 is an equivalent condition.

This transform requires that the add instruction be annotated with nsw.

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
    llvm/trunk/test/Transforms/InstCombine/icmp.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp?rev=221045&r1=221044&r2=221045&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp Sat Nov  1 04:09:51 2014
@@ -3015,6 +3015,12 @@ Instruction *InstCombiner::visitICmpInst
     if (BO1 && BO1->getOpcode() == Instruction::Add)
       C = BO1->getOperand(0), D = BO1->getOperand(1);
 
+    // icmp (X+cst) < 0 --> X < -cst
+    if (NoOp0WrapProblem && ICmpInst::isSigned(Pred) && match(Op1, m_Zero()))
+      if (ConstantInt *RHSC = dyn_cast_or_null<ConstantInt>(B))
+        if (!RHSC->isMinValue(/*isSigned=*/true))
+          return new ICmpInst(Pred, A, ConstantExpr::getNeg(RHSC));
+
     // icmp (X+Y), X -> icmp Y, 0 for equalities or if there is no overflow.
     if ((A == Op1 || B == Op1) && NoOp0WrapProblem)
       return new ICmpInst(Pred, A == Op1 ? B : A,

Modified: llvm/trunk/test/Transforms/InstCombine/icmp.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/icmp.ll?rev=221045&r1=221044&r2=221045&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/icmp.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/icmp.ll Sat Nov  1 04:09:51 2014
@@ -1479,3 +1479,35 @@ define i1 @shl_ap1_non_zero_ap2_non_zero
  %cmp = icmp eq i32 %shl, 50
  ret i1 %cmp
 }
+
+; CHECK-LABEL: @icmp_sgt_zero_add_nsw
+; CHECK-NEXT: icmp sgt i32 %a, -1
+define i1 @icmp_sgt_zero_add_nsw(i32 %a) {
+ %add = add nsw i32 %a, 1
+ %cmp = icmp sgt i32 %add, 0
+ ret i1 %cmp
+}
+
+; CHECK-LABEL: @icmp_sge_zero_add_nsw
+; CHECK-NEXT: icmp sgt i32 %a, -2
+define i1 @icmp_sge_zero_add_nsw(i32 %a) {
+ %add = add nsw i32 %a, 1
+ %cmp = icmp sge i32 %add, 0
+ ret i1 %cmp
+}
+
+; CHECK-LABEL: @icmp_slt_zero_add_nsw
+; CHECK-NEXT: icmp slt i32 %a, -1
+define i1 @icmp_slt_zero_add_nsw(i32 %a) {
+ %add = add nsw i32 %a, 1
+ %cmp = icmp slt i32 %add, 0
+ ret i1 %cmp
+}
+
+; CHECK-LABEL: @icmp_sle_zero_add_nsw
+; CHECK-NEXT: icmp slt i32 %a, 0
+define i1 @icmp_sle_zero_add_nsw(i32 %a) {
+ %add = add nsw i32 %a, 1
+ %cmp = icmp sle i32 %add, 0
+ ret i1 %cmp
+}