[InstCombine] Combine no-wrap sub and icmp w/ constant.

Teach InstCombine the transformation `(icmp P (sub nuw|nsw C2, Y), C) -> (icmp swap(P) Y, C2-C)`

Reviewers: majnemer, apilipenko, sanjoy, spatel, lebedev.ri

Reviewed By: lebedev.ri

Subscribers: dmgreen, lebedev.ri, nikic, hiraditya, JDevlieghere, jfb, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59916

llvm-svn: 357674

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index dc26725..7b58928 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2321,6 +2321,16 @@ Instruction *InstCombiner::foldICmpSubConstant(ICmpInst &Cmp,
                                                const APInt &C) {
   Value *X = Sub->getOperand(0), *Y = Sub->getOperand(1);
   ICmpInst::Predicate Pred = Cmp.getPredicate();
+  const APInt *C2;
+  APInt SubResult;
+
+  // (icmp P (sub nuw|nsw C2, Y), C) -> (icmp swap(P) Y, C2-C)
+  if (match(X, m_APInt(C2)) &&
+      ((Cmp.isUnsigned() && Sub->hasNoUnsignedWrap()) ||
+       (Cmp.isSigned() && Sub->hasNoSignedWrap())) &&
+      !subWithOverflow(SubResult, *C2, C, Cmp.isSigned()))
+    return new ICmpInst(Cmp.getSwappedPredicate(), Y,
+                        ConstantInt::get(Y->getType(), SubResult));
 
   // The following transforms are only worth it if the only user of the subtract
   // is the icmp.
@@ -2345,7 +2355,6 @@ Instruction *InstCombiner::foldICmpSubConstant(ICmpInst &Cmp,
       return new ICmpInst(ICmpInst::ICMP_SLE, X, Y);
   }
 
-  const APInt *C2;
   if (!match(X, m_APInt(C2)))
     return nullptr;
 

diff --git a/llvm/test/Transforms/InstCombine/icmp-sub.ll b/llvm/test/Transforms/InstCombine/icmp-sub.ll
index e9fbec9..c66581b 100644 (file)
--- a/llvm/test/Transforms/InstCombine/icmp-sub.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-sub.ll
@@ -3,8 +3,7 @@
 
 define i1 @test_nuw_and_unsigned_pred(i64 %x) {
 ; CHECK-LABEL: @test_nuw_and_unsigned_pred(
-; CHECK-NEXT:    [[Y:%.*]] = sub nuw i64 10, [[X:%.*]]
-; CHECK-NEXT:    [[Z:%.*]] = icmp ult i64 [[Y]], 3
+; CHECK-NEXT:    [[Z:%.*]] = icmp ugt i64 [[X:%.*]], 7
 ; CHECK-NEXT:    ret i1 [[Z]]
 ;
   %y = sub nuw i64 10, %x
@@ -14,8 +13,7 @@ define i1 @test_nuw_and_unsigned_pred(i64 %x) {
 
 define i1 @test_nsw_and_signed_pred(i64 %x) {
 ; CHECK-LABEL: @test_nsw_and_signed_pred(
-; CHECK-NEXT:    [[Y:%.*]] = sub nsw i64 3, [[X:%.*]]
-; CHECK-NEXT:    [[Z:%.*]] = icmp sgt i64 [[Y]], 10
+; CHECK-NEXT:    [[Z:%.*]] = icmp slt i64 [[X:%.*]], -7
 ; CHECK-NEXT:    ret i1 [[Z]]
 ;
   %y = sub nsw i64 3, %x
@@ -25,8 +23,7 @@ define i1 @test_nsw_and_signed_pred(i64 %x) {
 
 define i1 @test_nuw_nsw_and_unsigned_pred(i64 %x) {
 ; CHECK-LABEL: @test_nuw_nsw_and_unsigned_pred(
-; CHECK-NEXT:    [[Y:%.*]] = sub nuw nsw i64 10, [[X:%.*]]
-; CHECK-NEXT:    [[Z:%.*]] = icmp ult i64 [[Y]], 4
+; CHECK-NEXT:    [[Z:%.*]] = icmp ugt i64 [[X:%.*]], 6
 ; CHECK-NEXT:    ret i1 [[Z]]
 ;
   %y = sub nuw nsw i64 10, %x
@@ -36,8 +33,7 @@ define i1 @test_nuw_nsw_and_unsigned_pred(i64 %x) {
 
 define i1 @test_nuw_nsw_and_signed_pred(i64 %x) {
 ; CHECK-LABEL: @test_nuw_nsw_and_signed_pred(
-; CHECK-NEXT:    [[Y:%.*]] = sub nuw nsw i64 10, [[X:%.*]]
-; CHECK-NEXT:    [[Z:%.*]] = icmp slt i64 [[Y]], 3
+; CHECK-NEXT:    [[Z:%.*]] = icmp sgt i64 [[X:%.*]], 7
 ; CHECK-NEXT:    ret i1 [[Z]]
 ;
   %y = sub nuw nsw i64 10, %x