[llvm] r353911 - [LoopSimplifyCFG] Re-enable const branch folding by default

[Roman Tereshin via llvm-commits]

Hi Max,

Looks like this doesn’t collaborate with MemorySSA very well:

./bin/opt -loop-simplifycfg loop-simplifycfg-term-folding-AND-mssa-loop-dependency.ll -enable-mssa-loop-dependency=true -verify-memoryssa=true -enable-loop-simplifycfg-term-folding=true -S -o -
Assertion failed: (find(predecessors(&B), Phi->getIncomingBlock(I)) != pred_end(&B) && "Incoming phi block not a block predecessor"), function verifyDefUses, file ../lib/Analysis/MemorySSA.cpp, line 1947.

The IR is:

define void @main() {
entry:
  br label %for.body

for.body:                                         ; preds = %exit, %entry
  br label %switch.bb

switch.bb:                                        ; preds = %for.body
  switch i2 1, label %default.bb [
    i2 1, label %case.bb
  ]

case.bb:                                          ; preds = %switch
  br label %exit

default.bb:                                       ; preds = %switch
  unreachable

exit:                                             ; preds = %case.bb
  call void @foo()
  br label %for.body
}

declare void @foo()


Thanks,
Roman

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> On Feb 12, 2019, at 10:12 PM, Max Kazantsev via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> 
> Author: mkazantsev
> Date: Tue Feb 12 22:12:48 2019
> New Revision: 353911
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=353911&view=rev
> Log:
> [LoopSimplifyCFG] Re-enable const branch folding by default
> 
> Known underlying bugs have been fixed, intensive fuzz testing did not
> find any new problems. Re-enabling by default. Feel free to revert if
> it causes any functional failures.
> 
> Modified:
>    llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
> 
> Modified: llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp?rev=353911&r1=353910&r2=353911&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp Tue Feb 12 22:12:48 2019
> @@ -41,7 +41,7 @@ using namespace llvm;
> #define DEBUG_TYPE "loop-simplifycfg"
> 
> static cl::opt<bool> EnableTermFolding("enable-loop-simplifycfg-term-folding",
> -                                       cl::init(false));
> +                                       cl::init(true));
> 
> STATISTIC(NumTerminatorsFolded,
>           "Number of terminators folded to unconditional branches");
> 
> 
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