[llvm-dev] DSE: Remove useless stores between malloc & memset

[Dávid Bolvanský via llvm-dev]

"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
>
>
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