AMDGPU: test for uniformity of branch instruction, not its condition

[pendingchaos]

Summary

If a divergent branch instruction is marked as divergent by propagation
rule 2 in DivergencePropagator::exploreSyncDependency() and its condition
is uniform, that branch would incorrectly be assumed to be uniform.

[pendingchaos]

Not yet sure why the condition was not marked as divergent, but I think it was intentional.
Also not sure why the code was like this in the first place, but the new code seems to work fine.

[arsenm]

Needs a test

[pendingchaos]

Adds a test.

[arsenm]

test/CodeGen/AMDGPU/divergent-branch-uniform-condition.ll

8 ↗

(On Diff #180477)

Wrong check prefix

9–11 ↗

(On Diff #180477)

The checks are too thin. Maybe for this you should just use update_llc_test_checks?

[pendingchaos]

Makes use of assertions generated by update_llc_test_checks.

[arsenm]

LGTM

lib/Target/AMDGPU/AMDGPUAnnotateUniformValues.cpp

auto Q = MDR->getPointerDependencyFrom(MemoryLocation(Ptr), true,
                                            StartIt, BB, Load);
     if (Q.isClobber() || Q.isUnknown())
       return true;
   }
   return false;
 }
 
 void AMDGPUAnnotateUniformValues::visitBranchInst(BranchInst &I) {
-  if (I.isUnconditional())
-    return;
-
-  Value *Cond = I.getCondition();
-  if (!DA->isUniform(Cond))
-    return;
-
+  if (DA->isUniform(&I))
     setUniformMetadata(I.getParent()->getTerminator());
 }
 
 void AMDGPUAnnotateUniformValues::visitLoadInst(LoadInst &I) {
   Value *Ptr = I.getPointerOperand();
   if (!DA->isUniform(Ptr))
     return;
   auto isGlobalLoad = [&](LoadInst &Load)->bool {
     return Load.getPointerAddressSpace() == AMDGPUAS::GLOBAL_ADDRESS;

lib/Target/AMDGPU/SIAnnotateControlFlow.cpp

bool SIAnnotateControlFlow::doInitialization(Module &M) {
   Loop = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_loop);
   EndCf = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_end_cf);
   return false;
 }
 
 /// Is the branch condition uniform or did the StructurizeCFG pass
 /// consider it as such?
 bool SIAnnotateControlFlow::isUniform(BranchInst *T) {
-  return DA->isUniform(T->getCondition()) ||
+  return DA->isUniform(T) ||
          T->getMetadata("structurizecfg.uniform") != nullptr;
 }
 
 /// Is BB the last block saved on the stack ?
 bool SIAnnotateControlFlow::isTopOfStack(BasicBlock *BB) {
   return !Stack.empty() && Stack.back().first == BB;
 }