Only unswitch loops with uniform conditions

Loop unswitching can be extremely harmful for a SIMT target. In case
if hoisted condition is not uniform a SIMT machine will execute both
clones of a loop sequentially. Therefor LoopUnswitch checks if the
condition is non-divergent.

Since DivergenceAnalysis adds an expensive PostDominatorTree analysis
not needed for non-SIMT targets a new option is added to avoid unneded
analysis initialization. The method getAnalysisUsage is called when
TargetTransformInfo is not yet available and we cannot use it here.
For that reason a new field DivergentTarget is added to PassManagerBuilder
to control the behavior and set this field from a target.

Differential Revision: https://reviews.llvm.org/D30796

llvm-svn: 298104

1 files changed, 2 insertions, 0 deletions

diff --git a/llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp b/llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp
index f5973c17498..48442bcf2f1 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp
@@ -216,6 +216,8 @@ StringRef AMDGPUTargetMachine::getFeatureString(const Function &F) const {
 }
 
 void AMDGPUTargetMachine::adjustPassManager(PassManagerBuilder &Builder) {
+  Builder.DivergentTarget = true;
+
   bool Internalize = InternalizeSymbols &&
                      (getOptLevel() > CodeGenOpt::None) &&
                      (getTargetTriple().getArch() == Triple::amdgcn);