[llvm] r329047 - [SCEV] Make computeExitLimit more simple and more powerful
Maxim Kazantsev via llvm-commits
llvm-commits at lists.llvm.org
Sun Apr 22 18:25:40 PDT 2018
Hi Chandler,
Thank you very much for getting me informed! I'd be happy to help to deal with this issue. Please provide exact steps for reproducing this issue once you track them down (I don't typically work with Clang and might need some links or instructions how to build the configuration you are mentioning).
As for my suspicions (it is just a shot in the dark, I don't have solid arguments that this is exactly what is going on) is that some pass, likely close to backend, is misusing SCEV. We can end up with memory corruption if some optimization doesn't invoke "forgetLoop" for a loop it might have modified somehow. The old code could be just accidentally correct just because we could not compute anything for a loop which we failed to invalidate. In this case, it is something that must be fixed.
Fortunately, there is not so many places where forgetLoop is used. I will now go through them and try to see what could be wrong in how it's done there. With the patch under suspicion, we calculate exit counts for more loops now.
Please keep me informed on what you find!
Best regards,
Max
From: Chandler Carruth [mailto:chandlerc at gmail.com]
Sent: Saturday, April 21, 2018 10:20 AM
To: Maxim Kazantsev <max.kazantsev at azul.com>; Kit Barton <kbarton at ca.ibm.com>; Eric Christopher <echristo at google.com>; Han Shen <shenhan at google.com>; Hal Finkel <hfinkel at anl.gov>; dlj at google.com
Cc: llvm-commits at lists.llvm.org
Subject: Re: [llvm] r329047 - [SCEV] Make computeExitLimit more simple and more powerful
This doesn't have anything to do with ASan FWIW, that just enables -O1.
Essentially, we're seeing this test fail on a "normal" bootstrapped Clang with -O1 (or -O2) after this revision. But weirdly, the 3-stage PPC build bot isn't failing. Trying to figure out why next.
On Fri, Apr 20, 2018 at 7:20 PM Chandler Carruth <chandlerc at gmail.com<mailto:chandlerc at gmail.com>> wrote:
In case it wasn't clear, I have no reason to suspect the code in this commit is wrong. We've looked at it and the code looks totally fine.
Much more likely that it is exposing some underlying bug somewhere much like the one fixed here: https://reviews.llvm.org/D44818
On Fri, Apr 20, 2018 at 7:19 PM Chandler Carruth <chandlerc at gmail.com<mailto:chandlerc at gmail.com>> wrote:
We're seeing a miscompile with this that is ... really frustrating to pin down.
Specifically, it appears that if you build Clang itself using a Clang built after this revision (so a stage2 Clang binary in bootstrap parlance), and specifically build Clang itself with ASan enabled targeting PPC (no other architectures we test in this way appear to be impacted), and then use that asan+ppc stage2 Clang binary to run a specific OpenMP test (clang/test/OpenMP:for_simd_codegen.cpp), that test will fail. It will specifically fail the third RUN line with the output:
error: 'error' diagnostics seen but not expected:
(frontend): malformed or corrupted AST file: 'could not decompress embedded file contents: zlib error: Z_DATA_ERROR'
It is possible we have a miscompiled zlib after this revision? Haven't yet pinned down whether the *write* of the PCH just above it wrote corrupt data, or the *read* failed to read perfectly correct data.
Sadly, I don't have access to a PPC machine to easily reproduce this and test these things out. We just have an automated build that happens to use this configuration and spits out the failure. =/
CC-ing various folks who may be able to help try to reproduce this usefully.
Somewhat concerning is that this is the *only* failure we've seen testing past this revision. No other platform, no other test. Doesn't fail w/o ASan, etc etc. But it also is really reliable on our end.
If anyone is trying to reproduce this, happy to chat with them on IRC and try to give the exact commands used at each stage of this.
-Chandler
PS: Huge props to +David Jones<mailto:dlj at google.com> for pinning down the cause of this.
On Mon, Apr 2, 2018 at 11:00 PM Max Kazantsev via llvm-commits <llvm-commits at lists.llvm.org<mailto:llvm-commits at lists.llvm.org>> wrote:
Author: mkazantsev
Date: Mon Apr 2 22:57:19 2018
New Revision: 329047
URL: http://llvm.org/viewvc/llvm-project?rev=329047&view=rev
Log:
[SCEV] Make computeExitLimit more simple and more powerful
Current implementation of `computeExitLimit` has a big piece of code
the only purpose of which is to prove that after the execution of this
block the latch will be executed. What it currently checks is actually a
subset of situations where the exiting block dominates latch.
This patch replaces all these checks for simple particular cases with
domination check over loop's latch which is the only necessary condition
of taking the exiting block into consideration. This change allows to
calculate exact loop taken count for simple loops like
for (int i = 0; i < 100; i++) {
if (cond) {...} else {...}
if (i > 50) break;
. . .
}
Differential Revision: https://reviews.llvm.org/D44677
Reviewed By: efriedma
Modified:
llvm/trunk/lib/Analysis/ScalarEvolution.cpp
llvm/trunk/test/Analysis/ScalarEvolution/exact_iter_count.ll
Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=329047&r1=329046&r2=329047&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
+++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Mon Apr 2 22:57:19 2018
@@ -6884,63 +6884,12 @@ ScalarEvolution::computeBackedgeTakenCou
ScalarEvolution::ExitLimit
ScalarEvolution::computeExitLimit(const Loop *L, BasicBlock *ExitingBlock,
bool AllowPredicates) {
- // Okay, we've chosen an exiting block. See what condition causes us to exit
- // at this block and remember the exit block and whether all other targets
- // lead to the loop header.
- bool MustExecuteLoopHeader = true;
- BasicBlock *Exit = nullptr;
- for (auto *SBB : successors(ExitingBlock))
- if (!L->contains(SBB)) {
- if (Exit) // Multiple exit successors.
- return getCouldNotCompute();
- Exit = SBB;
- } else if (SBB != L->getHeader()) {
- MustExecuteLoopHeader = false;
- }
-
- // At this point, we know we have a conditional branch that determines whether
- // the loop is exited. However, we don't know if the branch is executed each
- // time through the loop. If not, then the execution count of the branch will
- // not be equal to the trip count of the loop.
- //
- // Currently we check for this by checking to see if the Exit branch goes to
- // the loop header. If so, we know it will always execute the same number of
- // times as the loop. We also handle the case where the exit block *is* the
- // loop header. This is common for un-rotated loops.
- //
- // If both of those tests fail, walk up the unique predecessor chain to the
- // header, stopping if there is an edge that doesn't exit the loop. If the
- // header is reached, the execution count of the branch will be equal to the
- // trip count of the loop.
- //
- // More extensive analysis could be done to handle more cases here.
- //
- if (!MustExecuteLoopHeader && ExitingBlock != L->getHeader()) {
- // The simple checks failed, try climbing the unique predecessor chain
- // up to the header.
- bool Ok = false;
- for (BasicBlock *BB = ExitingBlock; BB; ) {
- BasicBlock *Pred = BB->getUniquePredecessor();
- if (!Pred)
- return getCouldNotCompute();
- TerminatorInst *PredTerm = Pred->getTerminator();
- for (const BasicBlock *PredSucc : PredTerm->successors()) {
- if (PredSucc == BB)
- continue;
- // If the predecessor has a successor that isn't BB and isn't
- // outside the loop, assume the worst.
- if (L->contains(PredSucc))
- return getCouldNotCompute();
- }
- if (Pred == L->getHeader()) {
- Ok = true;
- break;
- }
- BB = Pred;
- }
- if (!Ok)
- return getCouldNotCompute();
- }
+ assert(L->contains(ExitingBlock) && "Exit count for non-loop block?");
+ // If our exiting block does not dominate the latch, then its connection with
+ // loop's exit limit may be far from trivial.
+ const BasicBlock *Latch = L->getLoopLatch();
+ if (!Latch || !DT.dominates(ExitingBlock, Latch))
+ return getCouldNotCompute();
bool IsOnlyExit = (L->getExitingBlock() != nullptr);
TerminatorInst *Term = ExitingBlock->getTerminator();
@@ -6955,9 +6904,19 @@ ScalarEvolution::computeExitLimit(const
/*ControlsExit=*/IsOnlyExit, AllowPredicates);
}
- if (SwitchInst *SI = dyn_cast<SwitchInst>(Term))
+ if (SwitchInst *SI = dyn_cast<SwitchInst>(Term)) {
+ // For switch, make sure that there is a single exit from the loop.
+ BasicBlock *Exit = nullptr;
+ for (auto *SBB : successors(ExitingBlock))
+ if (!L->contains(SBB)) {
+ if (Exit) // Multiple exit successors.
+ return getCouldNotCompute();
+ Exit = SBB;
+ }
+ assert(Exit && "Exiting block must have at least one exit");
return computeExitLimitFromSingleExitSwitch(L, SI, Exit,
/*ControlsExit=*/IsOnlyExit);
+ }
return getCouldNotCompute();
}
Modified: llvm/trunk/test/Analysis/ScalarEvolution/exact_iter_count.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/ScalarEvolution/exact_iter_count.ll?rev=329047&r1=329046&r2=329047&view=diff
==============================================================================
--- llvm/trunk/test/Analysis/ScalarEvolution/exact_iter_count.ll (original)
+++ llvm/trunk/test/Analysis/ScalarEvolution/exact_iter_count.ll Mon Apr 2 22:57:19 2018
@@ -25,3 +25,37 @@ exit:
side.exit:
ret void
}
+
+define void @test_02(i1 %c) {
+
+; CHECK-LABEL: Determining loop execution counts for: @test_02
+; CHECK-NEXT: Loop %loop: <multiple exits> backedge-taken count is 50
+
+entry:
+ br label %loop
+
+loop:
+ %iv = phi i32 [ 0, %entry ], [ %iv.next, %backedge ]
+ br i1 %c, label %if.true, label %if.false
+
+if.true:
+ br label %merge
+
+if.false:
+ br label %merge
+
+merge:
+ %side.cond = icmp slt i32 %iv, 50
+ br i1 %side.cond, label %backedge, label %side.exit
+
+backedge:
+ %iv.next = add i32 %iv, 1
+ %loop.cond = icmp slt i32 %iv, 100
+ br i1 %loop.cond, label %loop, label %exit
+
+exit:
+ ret void
+
+side.exit:
+ ret void
+}
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