[llvm-dev] status of IPO/IPCP?

[Davide Italiano via llvm-dev]

On Tue, May 3, 2016 at 11:52 PM, Davide Italiano <davide at freebsd.org> wrote:
> On Tue, May 3, 2016 at 11:01 PM, Justin Bogner <mail at justinbogner.com> wrote:
>> Sean Silva via llvm-dev <llvm-dev at lists.llvm.org> writes:
>>> No tests fail with the patch below, so I would say it's pretty useless. It
>>> seems that the C bindings are the only user but we can probably just have them
>>> return IPSCCP instead.
>>
>> I don't necessarily think your conclusion is wrong, but the patch isn't
>> proving what you think it's proving. In fact, the below passes all tests
>> as well. When you call passes through `opt` they don't end up calling
>> through the createXYZPass path.
>>
>> diff --git a/lib/Transforms/IPO/IPConstantPropagation.cpp b/lib/Transforms/IPO/IPConstantPropagation.cpp
>> index b3ee499..8d70b98 100644
>> --- a/lib/Transforms/IPO/IPConstantPropagation.cpp
>> +++ b/lib/Transforms/IPO/IPConstantPropagation.cpp
>> @@ -253,7 +253,9 @@ char IPCP::ID = 0;
>>  INITIALIZE_PASS(IPCP, "ipconstprop",
>>                  "Interprocedural constant propagation", false, false)
>>
>> -ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); }
>> +ModulePass *llvm::createIPConstantPropagationPass() {
>> +  llvm_unreachable("fnord");
>> +}
>>
>
> I think this should do the job:
>
> $ grep -R 'ipconstprop' * | wc -l
>    7
> $ sed -i.bak 's/ipconstprop/ipsccp/' *
> $ grep -R 'ipconstprop' * | wc -l
>        0
>
> and two tests are actually failing :|
>
> Testing Time: 0.10s
> ********************
> Failing Tests (2):
>     LLVM :: Transforms/IPConstantProp/return-argument.ll
>     LLVM :: Transforms/IPConstantProp/return-constants.ll
>
>   Expected Passes    : 11
>   Unexpected Failures: 2
>
> I'll take a closer look tomorrow.
>
>

I analyzed this and realized there are some cases that are actually
not handled properly by SCCP but they are by the naive algorithm. For
example, in return-argument SCCP doesn't propagate the return value
correctly



-- 
Davide

"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare