[PATCH] D29620: [JumpThreading] Thread through guards

[Sanjoy Das via Phabricator via llvm-commits]

sanjoy added inline comments.


================
Comment at: llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp:2076
+    if (Parent == Pred2->getSinglePredecessor())
+      if (BranchInst *BI = dyn_cast<BranchInst>(Parent->getTerminator()))
+        for (auto &I : *BB)
----------------
mkazantsev wrote:
> mkazantsev wrote:
> > sanjoy wrote:
> > > sanjoy wrote:
> > > > mkazantsev wrote:
> > > > > sanjoy wrote:
> > > > > > mkazantsev wrote:
> > > > > > > anna wrote:
> > > > > > > > Hi Max, I think you need to check that branch is a conditional branch. Otherwise we can fail while trying to get the condition for the branch in `ThreadGuard` at line 2091.
> > > > > > > You're right, I need to add this check here.
> > > > > > I guess this is possible when `Pred1` and `Pred2` are the same, like:
> > > > > > 
> > > > > > 
> > > > > > ```
> > > > > > Parent:
> > > > > >   br label %pred
> > > > > > 
> > > > > > pred:
> > > > > >   switch ... 4 -> BB; 5 -> BB; ...
> > > > > > 
> > > > > > BB:
> > > > > >   ...
> > > > > > ```
> > > > > > 
> > > > > > 
> > > > > > Are there other bits in this patch that are wrong in that situation?
> > > > > > 
> > > > > I think another example could be
> > > > > 
> > > > >   Parent:
> > > > >     br %cond Pred, Pred
> > > > > 
> > > > >   Pred:
> > > > >     br BB
> > > > > 
> > > > >   BB
> > > > >     ...
> > > > > 
> > > > > So I would rather add both checks "branch is conditional" and pred1 != pred2.
> > > > I think `getSinglePredecessor` on `Pred` will return `nullptr` in that case.  `getUniquePredecessor` would have returned `Parent`.
> > > Also, you would not have had two preds for `BB`.
> > Agreed. However, in your example the discussed situation doesn't happen as well. The thing is that JumpTheading will merge Parent and pred blocks before it gets to BB and runs into that crash. I will try to construct a test that reproduces the problem.
> > 
> > Anna, do you have a simple test where this crash happens? I'd appreciate if you provided it.
> I was able to create a test that looks like
> 
>   define i32 @not_a_diamond1(i32 %a, i1 %cond1) {
>     br i1 %cond1, label %Pred, label %Exit
>   Pred:
>     switch i32 %a, label %Exit [
>       i32 10, label %Merge
>       i32 20, label %Merge
>     ]
>   Merge:
>     call void(i1, ...) @llvm.experimental.guard( i1 %cond1 )[ "deopt"() ]
>     br label %Exit
>   Exit:
>     ret i32 %a
>   }
> 
> Despite the condition is conditional, here pred1 = pred2, and the test is buggy.
It is easy to fool JumpThreading's block ordering though, to prevent it from collapsing blocks:

```
declare void @llvm.experimental.guard(i1, ...)

define void @f(i32 %a, i1 %c) {
entry:
  br label %parent

merge:
  call void(i1, ...) @llvm.experimental.guard(i1 %c)[ "deopt"() ]
  ret void

pred:
  switch i32 %a, label %exit [
    i32 10, label %merge
    i32 20, label %merge
  ]

parent:
  br label %pred

exit:
  ret void
}
```

on `./bin/opt -jump-threading` with @anna 's revert reverted crashes as:

```
Assertion failed: (isConditional() && "Cannot get condition of an uncond branch!"), function getCondition, file ../../include/llvm/IR/Instructions.h, line 2989.
0  opt                      0x00000001044890bc llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 60
1  opt                      0x0000000104489639 PrintStackTraceSignalHandler(void*) + 25
2  opt                      0x00000001044856a9 llvm::sys::RunSignalHandlers() + 425
3  opt                      0x0000000104489b72 SignalHandler(int) + 354
4  libsystem_platform.dylib 0x00007fffdfc3ebba _sigtramp + 26
5  libsystem_platform.dylib 000000000000000000 _sigtramp + 540808288
6  libsystem_c.dylib        0x00007fffdfac5420 abort + 129
7  libsystem_c.dylib        0x00007fffdfa8c893 basename_r + 0
8  opt                      0x0000000103e82db8 llvm::BranchInst::getCondition() const + 104
9  opt                      0x00000001040d4b39 llvm::JumpThreadingPass::ThreadGuard(llvm::BasicBlock*, llvm::IntrinsicInst*, llvm::BranchInst*) + 105
10 opt                      0x00000001040cd17f llvm::JumpThreadingPass::ProcessGuards(llvm::BasicBlock*) + 479
11 opt                      0x00000001040c8ecd llvm::JumpThreadingPass::ProcessBlock(llvm::BasicBlock*) + 541
12 opt                      0x00000001040c87a7 llvm::JumpThreadingPass::runImpl(llvm::Function&, llvm::TargetLibraryInfo*, llvm::LazyValueInfo*, bool, std::__1::unique_ptr<llvm::BlockFrequencyInfo, std::__1::default_delete<llvm::BlockFrequencyInfo> >, std::__1::unique_ptr<llvm::BranchProbabilityInfo, std::__1::default_delete<llvm::BranchProbabilityInfo> >) + 1959
13 opt                      0x00000001040d5e21 (anonymous namespace)::JumpThreading::runOnFunction(llvm::Function&) +2161
```


Repository:
  rL LLVM

https://reviews.llvm.org/D29620