Add support for llvm.is.constant intrinsic (PR4898)

This adds the llvm-side support for post-inlining evaluation of the
__builtin_constant_p GCC intrinsic.

Also fixed SCCPSolver::visitCallSite to not blow up when seeing a call
to a function where canConstantFoldTo returns true, and one of the
arguments is a struct.

Updated from patch initially by Janusz Sobczak.

Differential Revision: https://reviews.llvm.org/D4276

llvm-svn: 346322

2 files changed, 15 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
index 035844294f4..d5f066c2423 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -1450,6 +1450,14 @@ bool FastISel::selectIntrinsicCall(const IntrinsicInst *II) {
     updateValueMap(II, ResultReg);
     return true;
   }
+  case Intrinsic::is_constant: {
+    Constant *ResCI = ConstantInt::get(II->getType(), 0);
+    unsigned ResultReg = getRegForValue(ResCI);
+    if (!ResultReg)
+      return false;
+    updateValueMap(II, ResultReg);
+    return true;
+  }
   case Intrinsic::launder_invariant_group:
   case Intrinsic::strip_invariant_group:
   case Intrinsic::expect: {
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 55ca5eb7c4e..bf24d7f7562 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -5897,6 +5897,13 @@ SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I, unsigned Intrinsic) {
     setValue(&I, Res);
     return nullptr;
   }
+
+  case Intrinsic::is_constant:
+    // If this wasn't constant-folded away by now, then it's not a
+    // constant.
+    setValue(&I, DAG.getConstant(0, sdl, MVT::i1));
+    return nullptr;
+
   case Intrinsic::annotation:
   case Intrinsic::ptr_annotation:
   case Intrinsic::launder_invariant_group: