[llvm-dev] Path condition propagation

[Carlos Liam via llvm-dev]

That seems ominous; should I not bother?

 - CL

> On Jul 3, 2016, at 5:58 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
> 
> PropagateEquality in gvn.cpp
> 
> However, if you are going to do it, remember the goal is to make the code simpler and easier, not just  pile on to the current mess to catch more cases :)
> 
>> On Mon, Jul 4, 2016, 7:51 AM Carlos Liam <carlos at aarzee.me> wrote:
>> Where would I look to change the equality propagation?
>> 
>> 
>>  - CL
>> 
>>> On Jun 30, 2016, at 11:45 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
>>> 
>>> The current gvn equality propagation is not powerful enough to get this because it doesn't try to infer values in predicates based on other predicates,   so it never realizes a>b -> a !=b in a useful way.
>>> 
>>> It otherwise would get this
>>> 
>>>> On Thu, Jun 30, 2016, 7:41 PM Sean Silva <chisophugis at gmail.com> wrote:
>>>>> On Thu, Jun 30, 2016 at 6:45 PM, Daniel Berlin via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>>> 
>>>>> 
>>>>>> On Thu, Jun 30, 2016 at 6:09 PM, Carlos Liam via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>>>> Hi all,
>>>>>> 
>>>>>> Consider this C code:
>>>>>> 
>>>>>> #include <stdbool.h>
>>>>>> 
>>>>>> bool func(int n1, int n2, bool b) {
>>>>>>     bool greater = n1 > n2;
>>>>>>     if (greater && b) {
>>>>>>         if (n1 == n2) {
>>>>>>             return false; // unreachable
>>>>>>         }
>>>>>>     }
>>>>>>     return true;
>>>>>> }
>>>>>> 
>>>>>> The line marked unreachable cannot be reached, however currently LLVM does not optimize it out
>>>>> 
>>>>> ?????
>>>>> Yes it does.
>>>> 
>>>> It seems like we get this almost by accident though. I find that I need `-mem2reg -instcombine -simplifycfg -instcombine` (on clang -O0 IR; the `-mem2reg -instcombine` are just cleanup) and essentially it boils down to simplifycfg merging everything into a single branch-free expression and then instcombine algebraically merging the comparisons.
>>>> 
>>>> A small modification defeats LLVM's optimizer:
>>>> 
>>>> bool func(int n1, int n2, bool b) {
>>>>     bool greater = n1 > n2;
>>>>     if (greater && b) {
>>>>         foo();
>>>> 
>>>>         if (n1 == n2) {
>>>>             return false; // unreachable
>>>>         }
>>>>     }
>>>>     return true;
>>>> }
>>>> 
>>>> In this case, simplifycfg doesn't go wild merging everything into a single branch-free expression and so we don't get it.
>>>> 
>>>> 
>>>> CorrelatedValuePropagation doesn't get this because its processCmp is quite weak (it bails out if one operand isn't a constant). JumpThreading is the only other pass that uses LazyValueInfo and it can't fold this since it can't thread a jump around the side-effecting `foo()` call.
>>>> 
>>>> I'm not familiar with GVN but it doesn't seem to help for this modified test case either.
>>>> 
>>>> Carlos, in answer to your original question, you may want to see if you can make LLVM get this case by modifying processCmp in lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
>>>> 
>>>> -- Sean Silva
>>>>  
>>>>> [dannyb at dannyb-macbookpro3 18:39:18] ~/sources/llvm (git-svn)-[newgvn-predicates]- :( $ clang -c -emit-llvm ~/greater.c -O1
>>>>> [dannyb at dannyb-macbookpro3 18:39:22] ~/sources/llvm (git-svn)-[newgvn-predicates]- :) $ debug-build/bin/llvm-dis greater.bc
>>>>> [dannyb at dannyb-macbookpro3 18:39:24] ~/sources/llvm (git-svn)-[newgvn-predicates]- :) $ cat greater.ll
>>>>> ; Function Attrs: norecurse nounwind readnone ssp uwtable
>>>>> define zeroext i1 @func(i32, i32, i1 zeroext) #0 {
>>>>>   ret i1 true
>>>>> } 
>>>>> 
>>>>> 
>>>>> opt -simplifycfg -instcombine does the same thing to it if you use -O0 with clang 
>>>>>>  
>>>>>> I believe this is because LLVM does not recognize that meeting path conditions like, for example, X && Y logically means that X is true and Y is true.
>>>>> 
>>>>> 
>>>>> Yes it does. See both GVN's propagateequality and correlatedvaluepropagation, among other things :)
>>>>> 
>>>>> In this case, simplifycfg +instcombine will do it
>>>>> 
>>>>> The new predicate support i'm building for GVN will also do it.
>>>>>  
>>>>>> 
>>>>>> I'm interested in creating a patch to remedy this; is there a file or function I should look at?
>>>>>> 
>>>>>> Thanks in advance.
>>>>>> 
>>>>>>  - CL
>>>>>> _______________________________________________
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>>>>>> llvm-dev at lists.llvm.org
>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>> 
>>>>> 
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