This patch builds on top of D13378 to handle constant condition.

With this patch, clang -O3 optimizes correctly providing > 1000x speedup on this artificial benchmark):

for (a=0; a<n; a++)
    for (b=0; b<n; b++)
        for (c=0; c<n; c++)
            for (d=0; d<n; d++)
                for (e=0; e<n; e++)
                    for (f=0; f<n; f++)
                        x++;
From test-suite/SingleSource/Benchmarks/Shootout/nestedloop.c

Reviewers: sanjoyd

Differential Revision: http://reviews.llvm.org/D13390

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 249431

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 9715d83fd10..caa0899f6a9 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -3904,6 +3904,11 @@ const SCEV *ScalarEvolution::createNodeForSelectOrPHI(Instruction *I,
                                                       Value *Cond,
                                                       Value *TrueVal,
                                                       Value *FalseVal) {
+  // Handle "constant" branch or select. This can occur for instance when a
+  // loop pass transforms an inner loop and moves on to process the outer loop.
+  if (auto *CI = dyn_cast<ConstantInt>(Cond))
+    return getSCEV(CI->isOne() ? TrueVal : FalseVal);
+
   // Try to match some simple smax or umax patterns.
   auto *ICI = dyn_cast<ICmpInst>(Cond);
   if (!ICI)