[llvm-dev] DSE: Remove useless stores between malloc & memset
Dávid Bolvanský via llvm-dev
llvm-dev at lists.llvm.org
Tue May 22 15:49:16 PDT 2018
It works with
MemoryLocation MemoryLocation::get(const CallInst *CI) {
AAMDNodes AATags;
CI->getAAMetadata(AATags);
const auto &DL = CI->getModule()->getDataLayout();
return MemoryLocation(CI, DL.getTypeStoreSize(CI->getType()), AATags);
}
Is it fine? :)
2018-05-22 23:56 GMT+02:00 Dávid Bolvanský <david.bolvansky at gmail.com>:
> 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/libpthr
>> ead.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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