[Reassociate] Don't canonicalize x + (-Constant * y) -> x - (Constant * y)..

..if the resulting subtract will be broken up later.  This can cause us to get
into an infinite loop.

x + (-5.0 * y)      -> x - (5.0 * y)       ; Canonicalize neg const
x - (5.0 * y)       -> x + (0 - (5.0 * y)) ; Break up subtract
x + (0 - (5.0 * y)) -> x + (-5.0 * y)      ; Replace 0-X with X*-1.

PR34078

llvm-svn: 311554

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/Reassociate.cpp b/llvm/lib/Transforms/Scalar/Reassociate.cpp
index 9825a590847..e0ef8cfe46b 100644
--- a/llvm/lib/Transforms/Scalar/Reassociate.cpp
+++ b/llvm/lib/Transforms/Scalar/Reassociate.cpp
@@ -1938,6 +1938,12 @@ Instruction *ReassociatePass::canonicalizeNegConstExpr(Instruction *I) {
   if (!User->isCommutative() && User->getOperand(1) != I)
     return nullptr;
 
+  // Don't canonicalize x + (-Constant * y) -> x - (Constant * y), if the
+  // resulting subtract will be broken up later.  This can get us into an
+  // infinite loop during reassociation.
+  if (UserOpcode == Instruction::FAdd && ShouldBreakUpSubtract(User))
+    return nullptr;
+
   // Change the sign of the constant.
   APFloat Val = CF->getValueAPF();
   Val.changeSign();