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[DA] propagate loop live-out values that get used in a branch 

Values that are uniform within a loop but appear divergent to uses
outside the loop are "tainted" so that such uses are marked
divergent. But if such a use is a branch, then it's divergence needs
to be propagated. The simplest way to do that is to put the branch
back in the main worklist so that it is processed appropriately.

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D81822
This commit is contained in:
Sameer Sahasrabuddhe 2020-06-17 08:14:50 +05:30
parent df9d64ed9c
commit d3963b3a5f
2 changed files with 66 additions and 9 deletions
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27
llvm/lib/Analysis/DivergenceAnalysis.cpp
View File

@ -184,6 +184,17 @@ bool DivergenceAnalysis::inRegion(const BasicBlock &BB) const {
return (!RegionLoop && BB.getParent() == &F) || RegionLoop->contains(&BB);
}
static bool usesLiveOut(const Instruction &I, const Loop *DivLoop) {
for (auto &Op : I.operands()) {
auto *OpInst = dyn_cast<Instruction>(&Op);
if (!OpInst)
continue;
if (DivLoop->contains(OpInst->getParent()))
return true;
}
return false;
}
// marks all users of loop-carried values of the loop headed by LoopHeader as
// divergent
void DivergenceAnalysis::taintLoopLiveOuts(const BasicBlock &LoopHeader) {
@ -227,16 +238,14 @@ void DivergenceAnalysis::taintLoopLiveOuts(const BasicBlock &LoopHeader) {
continue;
if (isDivergent(I))
continue;
if (!usesLiveOut(I, DivLoop))
continue;
for (auto &Op : I.operands()) {
auto *OpInst = dyn_cast<Instruction>(&Op);
if (!OpInst)
continue;
if (DivLoop->contains(OpInst->getParent())) {
markDivergent(I);
pushUsers(I);
break;
}
markDivergent(I);
if (I.isTerminator()) {
propagateBranchDivergence(I);
} else {
pushUsers(I);
}
}

48
llvm/test/Analysis/DivergenceAnalysis/AMDGPU/propagate-loop-live-out.ll Normal file
View File

@ -0,0 +1,48 @@
; RUN: opt -mtriple amdgcn-unknown-amdhsa -analyze -divergence -use-gpu-divergence-analysis %s | FileCheck %s
; CHECK: bb6:
; CHECK: DIVERGENT: %.126.i355.i = phi i1 [ false, %bb5 ], [ true, %bb4 ]
; CHECK: DIVERGENT: br i1 %.126.i355.i, label %bb7, label %bb8
; Function Attrs: nounwind readnone speculatable
declare i32 @llvm.amdgcn.workitem.id.x() #0
define protected amdgpu_kernel void @_Z23krnl_GPUITSFitterKerneli() {
bb0:
%i4 = call i32 @llvm.amdgcn.workitem.id.x()
%i5 = icmp eq i32 %i4, -1
br label %bb1
bb1: ; preds = %bb3, %bb0
%lsr.iv = phi i32 [ %i1, %bb3 ], [ 7, %bb0 ]
br i1 %i5, label %bb2, label %bb3
bb2: ; preds = %bb1
%lsr.iv.next = add nsw i32 %lsr.iv, -1
%i14 = icmp eq i32 %lsr.iv.next, 0
br label %bb3
bb3: ; preds = %bb2, %bb1
%i1 = phi i32 [ %lsr.iv.next, %bb2 ], [ 0, %bb1 ]
%i2 = phi i1 [ false, %bb2 ], [ true, %bb1 ]
%i3 = phi i1 [ %i14, %bb2 ], [ true, %bb1 ]
br i1 %i3, label %bb4, label %bb1
bb4: ; preds = %bb3
br i1 %i2, label %bb5, label %bb6
bb5: ; preds = %bb4
br label %bb6
bb6: ; preds = %bb5, %bb4
%.126.i355.i = phi i1 [ false, %bb5 ], [ true, %bb4 ]
br i1 %.126.i355.i, label %bb7, label %bb8
bb7: ; preds = %bb6
br label %bb8
bb8: ; preds = %bb7, %bb6
ret void
}
attributes #0 = { nounwind readnone speculatable }