AMDGPU: Avoid folding 2 constant operands into an SALU operation

Summary:
Catch the (admittedly unusual) case where SIFoldOperands attempts to fold 2
constant operands into the same SALU operation, with neither operand able to be
encoded as an inline constant.

Change-Id: Ibc48d662c9ffd8bbacd154976b0b1c257ace0927

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70896

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp b/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
index ca17ba8b722..f2c00ddce94 100644
--- a/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
+++ b/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
@@ -429,6 +429,29 @@ static bool tryAddToFoldList(SmallVectorImpl<FoldCandidate> &FoldList,
     return true;
   }
 
+  // Check the case where we might introduce a second constant operand to a
+  // scalar instruction
+  if (TII->isSALU(MI->getOpcode())) {
+    const MCInstrDesc &InstDesc = MI->getDesc();
+    const MCOperandInfo &OpInfo = InstDesc.OpInfo[OpNo];
+    const SIRegisterInfo &SRI = TII->getRegisterInfo();
+
+    // Fine if the operand can be encoded as an inline constant
+    if (OpToFold->isImm()) {
+      if (!SRI.opCanUseInlineConstant(OpInfo.OperandType) ||
+          !TII->isInlineConstant(*OpToFold, OpInfo)) {
+        // Otherwise check for another constant
+        for (unsigned i = 0, e = InstDesc.getNumOperands(); i != e; ++i) {
+          auto &Op = MI->getOperand(i);
+          if (OpNo != i &&
+              TII->isLiteralConstantLike(Op, OpInfo)) {
+            return false;
+          }
+        }
+      }
+    }
+  }
+
   appendFoldCandidate(FoldList, MI, OpNo, OpToFold);
   return true;
 }