fix a bug (possibly 8816) in the sadd forming xform: it isn't

profitable (or safe) to promote code when the add-with-constant
has other uses.

llvm-svn: 122175

2 files changed, 39 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 80497e9391a..376a38edda3 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1588,6 +1588,16 @@ Instruction *InstCombiner::visitICmpInstWithCastAndCast(ICmpInst &ICI) {
 static Instruction *ProcessUGT_ADDCST_ADD(ICmpInst &I, Value *A, Value *B,
                                           ConstantInt *CI2, ConstantInt *CI1,
                                           InstCombiner::BuilderTy *Builder) {
+  // The transformation we're trying to do here is to transform this into an
+  // llvm.sadd.with.overflow.  To do this, we have to replace the original add
+  // with a narrower add, and discard the add-with-constant that is part of the
+  // range check (if we can't eliminate it, this isn't profitable).
+  
+  // In order to eliminate the add-with-constant, the compare can be its only
+  // use.
+  Value *AddWithCst = I.getOperand(0);
+  if (!AddWithCst->hasOneUse()) return 0;
+
   const IntegerType *WideType = cast<IntegerType>(CI1->getType());
   unsigned WideWidth = WideType->getBitWidth();
   unsigned NarrowWidth = WideWidth / 2;
diff --git a/llvm/test/Transforms/InstCombine/overflow.ll b/llvm/test/Transforms/InstCombine/overflow.ll
index f3f8ca3d61d..e54d315cd04 100644
--- a/llvm/test/Transforms/InstCombine/overflow.ll
+++ b/llvm/test/Transforms/InstCombine/overflow.ll
@@ -1,6 +1,8 @@
 ; RUN: opt -S -instcombine < %s | FileCheck %s
 ; <rdar://problem/8558713>
 
+declare i32 @throwAnExceptionOrWhatever(...)
+
 ; CHECK: @test1
 define i32 @test1(i32 %a, i32 %b) nounwind ssp {
 entry:
@@ -24,4 +26,30 @@ if.end:
   ret i32 %conv9
 }
 
-declare i32 @throwAnExceptionOrWhatever(...)
+; CHECK: @test2
+; This form should not be promoted for two reasons: 1) it is unprofitable to
+; promote it since the add.off instruction has another use, and 2) it is unsafe
+; because the add-with-off makes the high bits of the original add live.
+define i32 @test2(i32 %a, i32 %b, i64* %P) nounwind ssp {
+entry:
+  %conv = sext i32 %a to i64
+  %conv2 = sext i32 %b to i64
+  %add = add nsw i64 %conv2, %conv
+  %add.off = add i64 %add, 2147483648
+  
+  store i64 %add.off, i64* %P
+  
+; CHECK-NOT: llvm.sadd.with.overflow
+  %0 = icmp ugt i64 %add.off, 4294967295
+  br i1 %0, label %if.then, label %if.end
+
+if.then:
+  %call = tail call i32 (...)* @throwAnExceptionOrWhatever() nounwind
+  br label %if.end
+
+if.end:
+  %conv9 = trunc i64 %add to i32
+; CHECK: ret i32
+  ret i32 %conv9
+}
+