fix another miscompile in the llvm.sadd formation logic: it wasn't
checking to see if the high bits of the original add result were dead.
Inserting a smaller add and zexting back to that size is not good enough.
This is likely to be the fix for 8816.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122177 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineCompares.cpp b/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 376a38e..1142686 100644
--- a/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1585,6 +1585,11 @@
/// ProcessUGT_ADDCST_ADD - The caller has matched a pattern of the form:
/// I = icmp ugt (add (add A, B), CI2), CI1
+/// If this is of the form:
+/// sum = a + b
+/// if (sum+128 >u 255)
+/// Then replace it with llvm.sadd.with.overflow.i8.
+///
static Instruction *ProcessUGT_ADDCST_ADD(ICmpInst &I, Value *A, Value *B,
ConstantInt *CI2, ConstantInt *CI1,
InstCombiner::BuilderTy *Builder) {
@@ -1595,9 +1600,41 @@
// In order to eliminate the add-with-constant, the compare can be its only
// use.
- Value *AddWithCst = I.getOperand(0);
+ Instruction *AddWithCst = cast<Instruction>(I.getOperand(0));
if (!AddWithCst->hasOneUse()) return 0;
-
+
+ // If CI2 is 2^7, 2^15, 2^31, then it might be an sadd.with.overflow.
+ if (!CI2->getValue().isPowerOf2()) return 0;
+ unsigned NewWidth = CI2->getValue().countTrailingZeros();
+ if (NewWidth != 7 && NewWidth != 15 && NewWidth != 31) return 0;
+
+ // The width of the new add formed is 1 more than the bias.
+ ++NewWidth;
+
+ // Check to see that CI1 is an all-ones value with NewWidth bits.
+ if (CI1->getBitWidth() == NewWidth ||
+ CI1->getValue() != APInt::getLowBitsSet(CI1->getBitWidth(), NewWidth))
+ return 0;
+
+ // In order to replace the original add with a narrower
+ // llvm.sadd.with.overflow, the only uses allowed are the add-with-constant
+ // and truncates that discard the high bits of the add. Verify that this is
+ // the case.
+ Instruction *OrigAdd = cast<Instruction>(AddWithCst->getOperand(0));
+ for (Value::use_iterator UI = OrigAdd->use_begin(), E = OrigAdd->use_end();
+ UI != E; ++UI) {
+ if (*UI == AddWithCst) continue;
+
+ // Only accept truncates for now. We would really like a nice recursive
+ // predicate like SimplifyDemandedBits, but which goes downwards the use-def
+ // chain to see which bits of a value are actually demanded. If the
+ // original add had another add which was then immediately truncated, we
+ // could still do the transformation.
+ TruncInst *TI = dyn_cast<TruncInst>(*UI);
+ if (TI == 0 ||
+ TI->getType()->getPrimitiveSizeInBits() > NewWidth) return 0;
+ }
+
const IntegerType *WideType = cast<IntegerType>(CI1->getType());
unsigned WideWidth = WideType->getBitWidth();
unsigned NarrowWidth = WideWidth / 2;
@@ -1630,8 +1667,6 @@
// If the pattern matches, truncate the inputs to the narrower type and
// use the sadd_with_overflow intrinsic to efficiently compute both the
// result and the overflow bit.
- Instruction *OrigAdd =
- cast<Instruction>(cast<Instruction>(I.getOperand(0))->getOperand(0));
Builder->SetInsertPoint(OrigAdd->getParent(),
BasicBlock::iterator(OrigAdd));