Do not try to optimize PHI nodes with incredibly high degree. This reduces SCCP

time from 615s to 1.49s on a large testcase that has a gigantic switch statement
that all of the blocks in the function go to (an intepreter).

llvm-svn: 12442

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/SCCP.cpp b/llvm/lib/Transforms/Scalar/SCCP.cpp
index 26c246df000..75be39704e1 100644
--- a/llvm/lib/Transforms/Scalar/SCCP.cpp
+++ b/llvm/lib/Transforms/Scalar/SCCP.cpp
@@ -500,6 +500,13 @@ void SCCP::visitPHINode(PHINode &PN) {
     return;  // Quick exit
   }
 
+  // Super-extra-high-degree PHI nodes are unlikely to ever be marked constant,
+  // and slow us down a lot.  Just mark them overdefined.
+  if (PN.getNumIncomingValues() > 64) {
+    markOverdefined(PNIV, &PN);
+    return;
+  }
+
   // Look at all of the executable operands of the PHI node.  If any of them
   // are overdefined, the PHI becomes overdefined as well.  If they are all
   // constant, and they agree with each other, the PHI becomes the identical