[llvm-dev] Path condition propagation

[Sean Silva via llvm-dev]

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
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
>
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