[llvm] r356298 - AMDGPU: Fix a SIAnnotateControlFlow issue when there are multiple backedges.

[Changpeng Fang via llvm-commits]

Author: chfang
Date: Fri Mar 15 14:02:48 2019
New Revision: 356298

URL: http://llvm.org/viewvc/llvm-project?rev=356298&view=rev
Log:
AMDGPU: Fix a SIAnnotateControlFlow issue when there are multiple backedges.

Summary:
At the exit of the loop, the compiler uses a register to remember and accumulate
the number of threads that have already exited. When all active threads exit the
loop, this register is used to restore the exec mask, and the execution continues
for the post loop code.

When there is a "continue" in the loop, the compiler made a mistake to reset the
register to 0 when the "continue" backedge is taken. This will result in some
threads not executing the post loop code as they are supposed to.

This patch fixed the issue.

Reviewers:
  nhaehnle, arsenm

Differential Revision:
  https://reviews.llvm.org/D59312

Added:
    llvm/trunk/test/CodeGen/AMDGPU/si-annotatecfg-multiple-backedges.ll
Modified:
    llvm/trunk/lib/Target/AMDGPU/SIAnnotateControlFlow.cpp

Modified: llvm/trunk/lib/Target/AMDGPU/SIAnnotateControlFlow.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/AMDGPU/SIAnnotateControlFlow.cpp?rev=356298&r1=356297&r2=356298&view=diff
==============================================================================
--- llvm/trunk/lib/Target/AMDGPU/SIAnnotateControlFlow.cpp (original)
+++ llvm/trunk/lib/Target/AMDGPU/SIAnnotateControlFlow.cpp Fri Mar 15 14:02:48 2019
@@ -264,8 +264,17 @@ void SIAnnotateControlFlow::handleLoop(B
   Term->setCondition(BoolTrue);
   Value *Arg = handleLoopCondition(Cond, Broken, L, Term);
 
-  for (BasicBlock *Pred : predecessors(Target))
-    Broken->addIncoming(Pred == BB ? Arg : Int64Zero, Pred);
+  for (BasicBlock *Pred : predecessors(Target)) {
+    Value *PHIValue = Int64Zero;
+    if (Pred == BB) // Remember the value of the previous iteration.
+      PHIValue = Arg;
+    // If the backedge from Pred to Target could be executed before the exit
+    // of the loop at BB, it should not reset or change "Broken", which keeps
+    // track of the number of threads exited the loop at BB.
+    else if (L->contains(Pred) && DT->dominates(Pred, BB))
+      PHIValue = Broken;
+    Broken->addIncoming(PHIValue, Pred);
+  }
 
   Term->setCondition(CallInst::Create(Loop, Arg, "", Term));
 

Added: llvm/trunk/test/CodeGen/AMDGPU/si-annotatecfg-multiple-backedges.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AMDGPU/si-annotatecfg-multiple-backedges.ll?rev=356298&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/AMDGPU/si-annotatecfg-multiple-backedges.ll (added)
+++ llvm/trunk/test/CodeGen/AMDGPU/si-annotatecfg-multiple-backedges.ll Fri Mar 15 14:02:48 2019
@@ -0,0 +1,61 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -mtriple=amdgcn-- -S -structurizecfg -si-annotate-control-flow %s | FileCheck -check-prefix=OPT %s
+
+; This test is designed to check that the backedge from a prior block won't
+; reset the variable introduced to record and accumulate the number of threads
+; which have already exited the loop.
+
+define amdgpu_kernel void @multiple_backedges(i32 %arg, i32* %arg1) {
+; OPT-LABEL: @multiple_backedges(
+; OPT-NEXT:  entry:
+; OPT-NEXT:    [[TMP:%.*]] = tail call i32 @llvm.amdgcn.workitem.id.x()
+; OPT-NEXT:    [[TMP2:%.*]] = shl nsw i32 [[ARG:%.*]], 1
+; OPT-NEXT:    br label [[LOOP:%.*]]
+; OPT:       loop:
+; OPT-NEXT:    [[PHI_BROKEN1:%.*]] = phi i64 [ [[TMP7:%.*]], [[LOOP_END:%.*]] ], [ [[PHI_BROKEN1]], [[LOOP]] ], [ 0, [[ENTRY:%.*]] ]
+; OPT-NEXT:    [[PHI_BROKEN:%.*]] = phi i64 [ 0, [[LOOP_END]] ], [ [[TMP0:%.*]], [[LOOP]] ], [ 0, [[ENTRY]] ]
+; OPT-NEXT:    [[TMP4:%.*]] = phi i32 [ 0, [[ENTRY]] ], [ [[TMP5:%.*]], [[LOOP]] ], [ 0, [[LOOP_END]] ]
+; OPT-NEXT:    [[TMP5]] = add nsw i32 [[TMP4]], [[TMP]]
+; OPT-NEXT:    [[TMP6:%.*]] = icmp slt i32 [[ARG]], [[TMP5]]
+; OPT-NEXT:    [[TMP0]] = call i64 @llvm.amdgcn.if.break(i1 [[TMP6]], i64 [[PHI_BROKEN]])
+; OPT-NEXT:    [[TMP1:%.*]] = call i1 @llvm.amdgcn.loop(i64 [[TMP0]])
+; OPT-NEXT:    br i1 [[TMP1]], label [[LOOP_END]], label [[LOOP]]
+; OPT:       loop_end:
+; OPT-NEXT:    call void @llvm.amdgcn.end.cf(i64 [[TMP0]])
+; OPT-NEXT:    [[EXIT:%.*]] = icmp sgt i32 [[TMP5]], [[TMP2]]
+; OPT-NEXT:    [[TMP7]] = call i64 @llvm.amdgcn.if.break(i1 [[EXIT]], i64 [[PHI_BROKEN1]])
+; OPT-NEXT:    [[TMP3:%.*]] = call i1 @llvm.amdgcn.loop(i64 [[TMP7]])
+; OPT-NEXT:    br i1 [[TMP3]], label [[LOOP_EXIT:%.*]], label [[LOOP]]
+; OPT:       loop_exit:
+; OPT-NEXT:    call void @llvm.amdgcn.end.cf(i64 [[TMP7]])
+; OPT-NEXT:    [[TMP12:%.*]] = zext i32 [[TMP]] to i64
+; OPT-NEXT:    [[TMP13:%.*]] = getelementptr inbounds i32, i32* [[ARG1:%.*]], i64 [[TMP12]]
+; OPT-NEXT:    [[TMP14:%.*]] = addrspacecast i32* [[TMP13]] to i32 addrspace(1)*
+; OPT-NEXT:    store i32 [[TMP5]], i32 addrspace(1)* [[TMP14]], align 4
+; OPT-NEXT:    ret void
+;
+entry:
+  %tmp = tail call i32 @llvm.amdgcn.workitem.id.x()
+  %tmp2 = shl nsw i32 %arg, 1
+  br label %loop
+
+loop:
+  %tmp4 = phi i32 [ 0, %entry ], [ %tmp5, %loop ], [ 0, %loop_end ]
+  %tmp5 = add nsw i32 %tmp4, %tmp
+  %tmp6 = icmp slt i32 %arg, %tmp5
+  br i1 %tmp6, label %loop_end, label %loop
+
+loop_end:
+  %exit = icmp sgt i32 %tmp5, %tmp2
+  br i1 %exit, label %loop_exit, label %loop
+
+loop_exit:
+  %tmp12 = zext i32 %tmp to i64
+  %tmp13 = getelementptr inbounds i32, i32* %arg1, i64 %tmp12
+  %tmp14 = addrspacecast i32* %tmp13 to i32 addrspace(1)*
+  store i32 %tmp5, i32 addrspace(1)* %tmp14, align 4
+  ret void
+}
+
+; Function Attrs: nounwind readnone speculatable
+declare i32 @llvm.amdgcn.workitem.id.x()