[llvm] r228302 - R600/SI: Fix bug from insertion of llvm.SI.end.cf into loop headers
Tom Stellard
tom at stellard.net
Thu Feb 5 08:14:08 PST 2015
Hi Hans,
Is this patch OK to merge to the 3.6 branch?
I am the code owner and I approve this patch.
-Tom
On Thu, Feb 05, 2015 at 03:32:15PM -0000, Tom Stellard wrote:
> Author: tstellar
> Date: Thu Feb 5 09:32:15 2015
> New Revision: 228302
>
> URL: http://llvm.org/viewvc/llvm-project?rev=228302&view=rev
> Log:
> R600/SI: Fix bug from insertion of llvm.SI.end.cf into loop headers
>
> The llvm.SI.end.cf intrinsic is used to mark the end of if-then blocks,
> if-then-else blocks, and loops. It is responsible for updating the
> exec mask to re-enable threads that had been masked during the preceding
> control flow block. For example:
>
> s_mov_b64 exec, 0x3 ; Initial exec mask
> s_mov_b64 s[0:1], exec ; Saved exec mask
> v_cmpx_gt_u32 exec, s[2:3], v0, 0 ; llvm.SI.if
> do_stuff()
> s_or_b64 exec, exec, s[0:1] ; llvm.SI.end.cf
>
> The bug fixed by this patch was one where the llvm.SI.end.cf intrinsic
> was being inserted into the header of loops. This would happen when
> an if block terminated in a loop header and we would end up with
> code like this:
>
> s_mov_b64 exec, 0x3 ; Initial exec mask
> s_mov_b64 s[0:1], exec ; Saved exec mask
> v_cmpx_gt_u32 exec, s[2:3], v0, 0 ; llvm.SI.if
> do_stuff()
>
> LOOP: ; Start of loop header
> s_or_b64 exec, exec, s[0:1] ; llvm.SI.end.cf <-BUG: The exec mask has the
> same value at the beginning of each loop
> iteration.
> do_stuff();
> s_cbranch_execnz LOOP
>
> The fix is to create a new basic block before the loop and insert the
> llvm.SI.end.cf there. This way the exec mask is restored before the
> start of the loop instead of at the beginning of each iteration.
>
> Added:
> llvm/trunk/test/CodeGen/R600/endcf-loop-header.ll
> Modified:
> llvm/trunk/lib/Target/R600/SIAnnotateControlFlow.cpp
>
> Modified: llvm/trunk/lib/Target/R600/SIAnnotateControlFlow.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/R600/SIAnnotateControlFlow.cpp?rev=228302&r1=228301&r2=228302&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Target/R600/SIAnnotateControlFlow.cpp (original)
> +++ llvm/trunk/lib/Target/R600/SIAnnotateControlFlow.cpp Thu Feb 5 09:32:15 2015
> @@ -14,6 +14,7 @@
>
> #include "AMDGPU.h"
> #include "llvm/ADT/DepthFirstIterator.h"
> +#include "llvm/Analysis/LoopInfo.h"
> #include "llvm/IR/Constants.h"
> #include "llvm/IR/Dominators.h"
> #include "llvm/IR/Instructions.h"
> @@ -66,6 +67,8 @@ class SIAnnotateControlFlow : public Fun
> DominatorTree *DT;
> StackVector Stack;
>
> + LoopInfo *LI;
> +
> bool isTopOfStack(BasicBlock *BB);
>
> Value *popSaved();
> @@ -99,6 +102,7 @@ public:
> }
>
> void getAnalysisUsage(AnalysisUsage &AU) const override {
> + AU.addRequired<LoopInfoWrapperPass>();
> AU.addRequired<DominatorTreeWrapperPass>();
> AU.addPreserved<DominatorTreeWrapperPass>();
> FunctionPass::getAnalysisUsage(AU);
> @@ -277,10 +281,26 @@ void SIAnnotateControlFlow::handleLoop(B
>
> Term->setCondition(CallInst::Create(Loop, Arg, "", Term));
> push(Term->getSuccessor(0), Arg);
> -}
> -
> -/// \brief Close the last opened control flow
> +}/// \brief Close the last opened control flow
> void SIAnnotateControlFlow::closeControlFlow(BasicBlock *BB) {
> + llvm::Loop *L = LI->getLoopFor(BB);
> +
> + if (L && L->getHeader() == BB) {
> + // We can't insert an EndCF call into a loop header, because it will
> + // get executed on every iteration of the loop, when it should be
> + // executed only once before the loop.
> + SmallVector <BasicBlock*, 8> Latches;
> + L->getLoopLatches(Latches);
> +
> + std::vector<BasicBlock*> Preds;
> + for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI) {
> + if (std::find(Latches.begin(), Latches.end(), *PI) == Latches.end())
> + Preds.push_back(*PI);
> + }
> + BB = llvm::SplitBlockPredecessors(BB, Preds, "endcf.split", nullptr, DT,
> + LI, false);
> + }
> +
> CallInst::Create(EndCf, popSaved(), "", BB->getFirstInsertionPt());
> }
>
> @@ -288,6 +308,7 @@ void SIAnnotateControlFlow::closeControl
> /// recognize if/then/else and loops.
> bool SIAnnotateControlFlow::runOnFunction(Function &F) {
> DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
> + LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
>
> for (df_iterator<BasicBlock *> I = df_begin(&F.getEntryBlock()),
> E = df_end(&F.getEntryBlock()); I != E; ++I) {
>
> Added: llvm/trunk/test/CodeGen/R600/endcf-loop-header.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/R600/endcf-loop-header.ll?rev=228302&view=auto
> ==============================================================================
> --- llvm/trunk/test/CodeGen/R600/endcf-loop-header.ll (added)
> +++ llvm/trunk/test/CodeGen/R600/endcf-loop-header.ll Thu Feb 5 09:32:15 2015
> @@ -0,0 +1,34 @@
> +; RUN: llc < %s -march=amdgcn -mcpu=SI -verify-machineinstrs | FileCheck %s
> +
> +; This tests that the llvm.SI.end.cf intrinsic is not inserted into the
> +; loop block. This intrinsic will be lowered to s_or_b64 by the code
> +; generator.
> +
> +; CHECK-LABEL: {{^}}test:
> +
> +; This is was lowered from the llvm.SI.end.cf intrinsic:
> +; CHECK: s_or_b64 exec, exec
> +
> +; CHECK: [[LOOP_LABEL:[0-9A-Za-z_]+]]: ; %loop{{$}}
> +; CHECK-NOT: s_or_b64 exec, exec
> +; CHECK: s_cbranch_execnz [[LOOP_LABEL]]
> +define void @test(i32 addrspace(1)* %out, i32 %cond) {
> +entry:
> + %tmp0 = icmp eq i32 %cond, 0
> + br i1 %tmp0, label %if, label %loop
> +
> +if:
> + store i32 0, i32 addrspace(1)* %out
> + br label %loop
> +
> +loop:
> + %tmp1 = phi i32 [0, %entry], [0, %if], [%inc, %loop]
> + %inc = add i32 %tmp1, %cond
> + %tmp2 = icmp ugt i32 %inc, 10
> + br i1 %tmp2, label %done, label %loop
> +
> +done:
> + %tmp3 = getelementptr i32 addrspace(1)* %out, i64 1
> + store i32 %inc, i32 addrspace(1)* %tmp3
> + ret void
> +}
>
>
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