LoopIdiomRegognize vs Preserved


I'm having problems with the LoopIdiomRegognizer crashing on me with

An asserting value handle still pointed to this value!
UNREACHABLE executed at ../lib/IR/Value.cpp:695!

If I remove




everything goes well.

The C-code triggering this is

void foo(int a[10][10])
     int i, j, k;

     for (i = 0; i < 1; i++) {
         for (j = 0; j < 2; j++) {
             for (k = 0; k < 10; k++) {
                 a[j][k] = 42;

First LoopIdiomRecognize replaces the store in the inner loop with a memset in the outer loop, and later, when examining the outer loop it tries to replace that memset with an even bigger memset in the outermost loop. But then, when removing the "old" memset, the assert blows.

I don't know LoopIdiomRecognize very well at all, is it obvious that AAResultsWrapperPass and/or LoopInfoWrapperPass should not be preserved here?


Hi Mikael,

What is your compilation command to trig the assert? I am trying to reproduce your problem.

Thank you,


Hi Haicheng,

Originally I ran into this on our out-of-tree target but I managed to reproduce the crash on X86 as well now:

  build-all/bin/opt -S -sroa -loop-rotate -loop-deletion -licm -loop-idiom ../llvm/bugpoint-reduced-simplified.i8+.ll


While deleting: void %
An asserting value handle still pointed to this value!
UNREACHABLE executed at ../lib/IR/Value.cpp:696!
0 opt 0x0000000001752bc8 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1 opt 0x0000000001751376 llvm::sys::RunSignalHandlers() + 54
2 opt 0x00000000017537ca
3 libpthread.so.0 0x00007f1909a70340
4 libc.so.6 0x00007f1908c98cc9 gsignal + 57
5 libc.so.6 0x00007f1908c9c0d8 abort + 328
6 opt 0x000000000170cddd llvm::llvm_unreachable_internal(char const*, char const*, unsigned int) + 461
7 opt 0x000000000137bd3b llvm::ValueHandleBase::ValueIsDeleted(llvm::Value*) + 1051
8 opt 0x000000000137b5db llvm::Value::~Value() + 43
9 opt 0x0000000001322319 llvm::CallInst::~CallInst() + 9
10 opt 0x000000000131e676 llvm::Instruction::eraseFromParent() + 86
11 opt 0x00000000015e8d14
12 opt 0x00000000015e8630
13 opt 0x00000000015e4e6a
14 opt 0x0000000000f5404e llvm::LPPassManager::runOnFunction(llvm::Function&) + 1086
15 opt 0x000000000134a034 llvm::FPPassManager::runOnFunction(llvm::Function&) + 516
16 opt 0x000000000134a27b llvm::FPPassManager::runOnModule(llvm::Module&) + 43
17 opt 0x000000000134a757 llvm::legacy::PassManagerImpl::run(llvm::Module&) + 903
18 opt 0x000000000062aa9e main + 8782
19 libc.so.6 0x00007f1908c83ec5 __libc_start_main + 245
20 opt 0x0000000000618bcf
Stack dump:
0. Program arguments: build-all/bin/opt -S -sroa -loop-rotate -loop-deletion -licm -loop-idiom ../llvm/bugpoint-reduced-simplified.i8+.ll
1. Running pass 'Function Pass Manager' on module '../llvm/bugpoint-reduced-simplified.i8+.ll'.
2. Running pass 'Loop Pass Manager' on function '@set_array'
3. Running pass 'Recognize loop idioms' on basic block '%bb4'


bugpoint-reduced-simplified.i8+.ll (1.29 KB)

Thank you, Mikael. I can reproduce what you saw and am looking into it.
Just curious, why do you run loop-deletion before licm and loop-idiom?
The latter two can cause empty loops.




Thank you, Mikael. I can reproduce what you saw and am looking into it.


Just curious, why do you run loop-deletion before licm and loop-idiom?

As part of our internal testing we use Csmith to generate C-programs and then we run the compiler with random generated compiler flags on that input.

This bug was triggered in one of those runs, so the options to opt are not something we use in our normal pipeline, but used to find faults.


Hi Michael,

I figure out what is going on here. If you run in the order of
-loop-deletion -licm -loop-idiom, loop-deletion runs in the first function
pass, licm and loop-idiom run together in the second function pass (i.e.
loop-deletion processes all the three nested loops first, then licm on the
inner loop, loop-idiom on the inner loop, licm on the middle loop,
loop-idiom on the middle loop, ...). So, after loop-deletion,

1. In the innermost loop, licm has no optimization to do.

2. loop-idiom replaces the store of the innermost loop with a small memset
in the middle loop.

3. In the middle loop, LICM adds the small memset to a AliasSetTracker.
AliasSetTracker does not recognize memset and treats memset as an unknown
instruction. All unknown instructions are tagged with an Asserting

4. LICM assumes no other pass can modify the processed sub loop, but
loop-idiom replaces the small memset in the middle loop with the large
memset in the outer loop. When deleting the small memset, the Asserting
ValueHandle still exists and triggers the assertion.

If removing AU.addPreserved<AAResultsWrapperPass>(), licm and loop-idiom run
separately and the Asserting ValueHandle can be properly handled by LICM.
If you run in the normal loop pipeline (-licm -loop-idiom -loop-deletion),
licm and loop-idiom are also separated.

I think there are several issues here

1. loop-idiom preserves AAResults, but does not update it.
2. licm's assumption that no other loop pass interferes with licm's
processed subloops is not safe.
3. AliasSetTracker treats memset as an unknown instruction.



Hi Haicheng,

Thanks for the analysis!

So what do we do with this? Should I write a bug report and fill in what we know about the problem?


Hi Mikael,

I created a patch to fix this (http://reviews.llvm.org/D17303). Please