[llvm-dev] DSE: Remove useless stores between malloc & memset
Dávid Bolvanský via llvm-dev
llvm-dev at lists.llvm.org
Tue May 22 14:56:43 PDT 2018
Looks like there are many overloads for "get".
http://llvm.org/doxygen/MemoryLocation_8cpp_source.html
But nothing for CallInst. Any suggestions how to do a proper one? I will
look at it too.
2018-05-22 23:34 GMT+02:00 Dávid Bolvanský <david.bolvansky at gmail.com>:
> Full stack trace:
>
> opt: /home/xbolva00/LLVM/llvm/include/llvm/ADT/Optional.h:176: T*
> llvm::Optional<T>::getPointer() [with T = llvm::MemoryLocation]:
> Assertion `Storage.hasVal' failed.
> Stack dump:
> 0. Program arguments: opt aaa.ll -dse -S
> 1. Running pass 'Function Pass Manager' on module 'aaa.ll'.
> 2. Running pass 'Dead Store Elimination' on function '@calloc_strlen'
> LLVMSymbolizer: error reading file: No such file or directory
> #0 0x000056135ebe698a (opt+0x212198a)
> #1 0x000056135ebe4cf4 (opt+0x211fcf4)
> #2 0x000056135ebe4e32 (opt+0x211fe32)
> #3 0x00007f6e35b14150 __restore_rt (/lib/x86_64-linux-gnu/
> libpthread.so.0+0x13150)
> #4 0x00007f6e3481b0bb gsignal /build/glibc-itYbWN/glibc-2.
> 26/signal/../sysdeps/unix/sysv/linux/raise.c:51:0
> #5 0x00007f6e3481cf5d abort /build/glibc-itYbWN/glibc-2.
> 26/stdlib/abort.c:92:0
> #6 0x00007f6e34812f17 __assert_fail_base /build/glibc-itYbWN/glibc-2.
> 26/assert/assert.c:92:0
> #7 0x00007f6e34812fc2 (/lib/x86_64-linux-gnu/libc.so.6+0x2efc2)
> #8 0x000056135e962b80 (opt+0x1e9db80)
> #9 0x000056135e969260 (opt+0x1ea4260)
> #10 0x000056135e96a6e0 (opt+0x1ea56e0)
> #11 0x000056135e61d561 (opt+0x1b58561)
> #12 0x000056135e61d5d9 (opt+0x1b585d9)
> #13 0x000056135e61cbb7 (opt+0x1b57bb7)
> #14 0x000056135d175216 (opt+0x6b0216)
> #15 0x00007f6e348051c1 __libc_start_main /build/glibc-itYbWN/glibc-2.
> 26/csu/../csu/libc-start.c:342:0
> #16 0x000056135d1f404a (opt+0x72f04a)
>
>
> 2018-05-22 23:32 GMT+02:00 Friedman, Eli <efriedma at codeaurora.org>:
>
>> It looks like the memoryIsNotModifiedBetween assumes the second argument
>> is a store, or some other instruction supported by MemoryLocation::get. If
>> you're passing in something else, you'll have to compute the MemoryLocation
>> some other way.
>>
>> (Generally, if you're asking a question about an assertion, please
>> include the whole stack trace; it's hard to guess what's happening
>> otherwise.)
>>
>> -Eli
>>
>>
>> On 5/22/2018 2:16 PM, Dávid Bolvanský wrote:
>>
>> * if (isStringFromCalloc(Dst, TLI)) should be if
>> (!isStringFromCalloc(Dst, TLI))
>> but still asserting...
>>
>> 2018-05-22 23:06 GMT+02:00 Dávid Bolvanský <david.bolvansky at gmail.com>:
>>
>>> Can you help a bit?
>>>
>>> I try to work with DSE but I got the following assert:
>>> opt: /home/xbolva00/LLVM/llvm/include/llvm/ADT/Optional.h:176: T*
>>> llvm::Optional<T>::getPointer() [with T = llvm::MemoryLocation]:
>>> Assertion `Storage.hasVal' failed.
>>>
>>> static bool eliminateStrlen(CallInst *CI, BasicBlock::iterator &BBI,
>>> AliasAnalysis *AA, MemoryDependenceResults *MD,
>>> const DataLayout &DL, const TargetLibraryInfo *TLI,
>>> InstOverlapIntervalsTy &IOL,
>>> DenseMap<Instruction *, size_t> *InstrOrdering) {
>>>
>>> // Must be a strlen.
>>> LibFunc Func;
>>> Function *Callee = CI->getCalledFunction();
>>> if (!TLI->getLibFunc(*Callee, Func) || !TLI->has(Func) ||
>>> Func != LibFunc_strlen)
>>> return false;
>>>
>>> Value *Dst = CI->getOperand(0);
>>> Instruction *UnderlyingPointer = dyn_cast<Instruction>(GetUnderlyingObject(Dst, DL));
>>> if (!UnderlyingPointer)
>>> return false;
>>> if (isStringFromCalloc(Dst, TLI))
>>> return false;
>>> errs() << "before\n";
>>> if (memoryIsNotModifiedBetween(UnderlyingPointer, CI, AA)) { <--- CRASH
>>> errs() << "after\n";
>>> }
>>> return false;
>>> }
>>>
>>> Do you know what is wrong here? I followed the "example" (in eliminateNoopStore) how to use "memoryIsNotModifiedBetween".
>>>
>>> Thank you for advice
>>>
>>>
>>> 2018-05-21 21:06 GMT+02:00 Friedman, Eli <efriedma at codeaurora.org>:
>>>
>>>> memoryIsNotModifiedBetween is precisely the sort of expensive walk we
>>>> shouldn't be doing... I'm surprised it hasn't caused any serious issues
>>>> yet. Ideally, what we should be doing is using MemorySSA to find a
>>>> dependency from the memset: if the closest dependency is the malloc, there
>>>> aren't any stores between the memset and the malloc. (But we aren't using
>>>> MemorySSA in DSE yet; see https://reviews.llvm.org/D40480.)
>>>>
>>>> But yes, memoryIsNotModifiedBetween has the right meaning.
>>>>
>>>> -Eli
>>>>
>>>>
>>>> On 5/21/2018 7:48 AM, Dávid Bolvanský wrote:
>>>>
>>>> "memory accesses between the malloc and the memset without an
>>>> expensive linear scan of the block/function"
>>>>
>>>> (1) do you mean just use "memoryIsNotModifiedBetween" function in DSE
>>>> to check it?
>>>>
>>>> x = maloc(..);
>>>> memset(x, ...)
>>>>
>>>> (2) GetUnderlyingObject would give me Value * (from malloc) ?
>>>>
>>>> Also another case:
>>>>
>>>> memset(s, 0, len); // len > 1
>>>> return strlen(s); // optimize to 0
>>>>
>>>> (3) How to check memset and strlen pairs? I have a strlen call, I have
>>>> a "Value *" for "s". What is the best way to construct memset +
>>>> strlen pairs?
>>>>
>>>> (4) Can this be somehow generalized (and not only for strlen)? So
>>>> GetStringLength in ValueTracking would be taught about this info (string is
>>>> empty after memset)
>>>>
>>>> (5) new malloc memset folding / memset + strlen case should be
>>>> implemented in DSE, right?
>>>>
>>>> 2018-05-17 21:36 GMT+02:00 Friedman, Eli <efriedma at codeaurora.org>:
>>>>
>>>>> The fundamental problem with trying to do that sort of transform in
>>>>> instcombine is that you don't have access to MemoryDependenceAnalysis or
>>>>> MemorySSA; you need a data structure like that to figure out whether there
>>>>> are any memory accesses between the malloc and the memset without an
>>>>> expensive linear scan of the block/function. (You can sort of get around
>>>>> the problem in simple cases by adding arbitrary limits to the number of you
>>>>> scan, but it doesn't generalize well.)
>>>>>
>>>>> -Eli
>>>>>
>>>>>
>>>>> On 5/17/2018 12:17 PM, Dávid Bolvanský wrote:
>>>>>
>>>>> As we talked in https://reviews.llvm.org/D45344, the problem was dead
>>>>> stores. And I know why :D There was just -instcombine pass. I forgot to do
>>>>> -dse before -instcombine so this is why I did custom "store removal" code
>>>>> there.
>>>>>
>>>>> I would like to finish malloc + llvm.memset folding. Yes, you told you
>>>>> would like to see the whole foldMallocMemset in DSE but extend it for
>>>>> llvm.memset in InstCombine... is it really so bad to do?
>>>>> We have standard malloc + memset folding there, so a few new lines
>>>>> should not do bad things.
>>>>>
>>>>> If I reopen D45344, reupload patch with removed my custom "store
>>>>> removal" code, It could be ok, no? The patch as is worked/works for me for
>>>>> malloc + llvm.memset folding, I would just add -dse to tests to handle dead
>>>>> stores.
>>>>>
>>>>>
>>>>>
>>>>> 2018-05-17 21:00 GMT+02:00 Friedman, Eli <efriedma at codeaurora.org>:
>>>>>
>>>>>> On 5/17/2018 8:58 AM, Dávid Bolvanský via llvm-dev wrote:
>>>>>>
>>>>>>> Hello,
>>>>>>>
>>>>>>> I would like to find a way to do this removal properly. I found DSE
>>>>>>> and "eliminateNoopStore" can be useful for this thing.
>>>>>>>
>>>>>>> What I mean?
>>>>>>> int *test = malloc(15 * sizeof(int));
>>>>>>> test[10] = 12; < ----- remove this store
>>>>>>> memset(test,0,sizeof(int) * 15);
>>>>>>>
>>>>>>
>>>>>> This is classic dead store elimination, and it's already handled by
>>>>>> DSE. At least, we optimize the following testcase:
>>>>>>
>>>>>> #include <stdlib.h>
>>>>>> #include <string.h>
>>>>>> void bar(int*);
>>>>>> void f() {
>>>>>> int *test = malloc(15 * sizeof(int));
>>>>>> test[10] = 12;
>>>>>> memset(test,0,sizeof(int) * 15);
>>>>>> bar(test);
>>>>>> }
>>>>>>
>>>>>> You should be able to look at the existing code to understand how
>>>>>> it's handled (using MemoryDependenceAnalysis).
>>>>>>
>>>>>> -Eli
>>>>>>
>>>>>> --
>>>>>> Employee of Qualcomm Innovation Center, Inc.
>>>>>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a
>>>>>> Linux Foundation Collaborative Project
>>>>>>
>>>>>>
>>>>>
>>>>> --
>>>>> Employee of Qualcomm Innovation Center, Inc.
>>>>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>>>>>
>>>>>
>>>>
>>>> --
>>>> Employee of Qualcomm Innovation Center, Inc.
>>>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>>>>
>>>>
>>>
>>
>> --
>> Employee of Qualcomm Innovation Center, Inc.
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>>
>>
>
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