[llvm] [Uniformity] Remove dependence on CycleInfo in old pass manager (PR #76011)

[Jay Foad via llvm-commits]

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@@ -152,26 +152,34 @@ UniformityInfoWrapperPass::UniformityInfoWrapperPass() : FunctionPass(ID) {
 INITIALIZE_PASS_BEGIN(UniformityInfoWrapperPass, "uniformity",
                       "Uniformity Analysis", true, true)
 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
-INITIALIZE_PASS_DEPENDENCY(CycleInfoWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
 INITIALIZE_PASS_END(UniformityInfoWrapperPass, "uniformity",
                     "Uniformity Analysis", true, true)
 
 void UniformityInfoWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.setPreservesAll();
   AU.addRequired<DominatorTreeWrapperPass>();
-  AU.addRequiredTransitive<CycleInfoWrapperPass>();
   AU.addRequired<TargetTransformInfoWrapperPass>();
 }
 
 bool UniformityInfoWrapperPass::runOnFunction(Function &F) {
-  auto &cycleInfo = getAnalysis<CycleInfoWrapperPass>().getResult();
   auto &domTree = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
   auto &targetTransformInfo =
       getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
 
   m_function = &F;
-  m_uniformityInfo = UniformityInfo{domTree, cycleInfo, &targetTransformInfo};
+
+  // FIXME: CycleInfo is a transitive dependency for UniformityInfo, and we
+  // would have liked to say that CycleInfo is also preserved whenever
+  // UniformityInfo is preserved. But some passes like
+  // AMDGPUUnifyDivergentExitNodes do not update cycle membership and lists of
+  // cycle exits, since that facility is not available. The end result is that
+  // we cannot declare CycleInfo as a transitive depedency for UniformityInfo.
+  // Instead we recompute CycleInfo on demand. This is OK because it is rarely
+  // needed, and only affects the old pass manager.
+  m_cycleInfo.clear();
+  m_cycleInfo.compute(F);
+  m_uniformityInfo = UniformityInfo{domTree, m_cycleInfo, &targetTransformInfo};
----------------
jayfoad wrote:

The transitive dependency means that CycleInfo must be present and correct not just when UniformityInfo is run, but whenever it is queried. If UnifyDivergentExitNodes claims to preserve UniformityInfo but does not update CycleInfo then it sounds like there is a real problem there: as soon as UnifyDivergentExitNodes has made some changes, a query on UniformityInfo will no longer be reliable because CycleInfo will not be up to date. I don't see how computing CycleInfo in this "run" function fixes that problem.

https://github.com/llvm/llvm-project/pull/76011