[llvm-dev] Path condition propagation

[Carlos Liam via llvm-dev]

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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>>> 
>>> 
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