[LLVMdev] Use clang to generate LLVM IR with -O3, how to visit these load operations after -reg2mem

[David Blaikie]

On Tue, Apr 21, 2015 at 9:29 PM, Eric Lu <eirc.lew at gmail.com> wrote:
>
> Best Regards!
> Eric Lew
>
> On 周三, 4月 22, 2015 at 12:11 下午, David Blaikie <dblaikie at gmail.com> wrote:
>
> On Tue, Apr 21, 2015 at 8:34 PM, Eric Lu <eirc.lew at gmail.com> wrote:
>> Hi, all
>>
>> I want to instrument load/store operations in LLVM IR. And I find the LLVM
>> IR generated with flag -o3 is much more efficient than -o0, so I try to
>> instrument on these optimized LLVM IR, but, some load operations are
>> changed
>> to Phi loads. Then, I try the -reg2mem flag to remove these phi nodes.
>> The  problems are:
>> 1) I use visitLoadInst to collect these load operations, and it seems I
>> cannot reach these loads that are generated by reg2mem? What is the
>> reason?
>
> Clang doesn't generate optimized IR - so if your instrumentation is in
> Clang's IRGen (clang/lib/CodeGen) it won't see these. Clang generates
> the unoptimized IR (you can see it with -Xclang -disable-llvm-optzns)
>
> I do instrumentation in the opt of llvm. Does clang itself do some
> optimizations that speed up applications with -O3 compared to -O0?

A little bit, but not much.

>
>
>> 2) Is there any way to compile with -O3 but not generate phi nodes ?
>
> Not likely - without that it'd disable a lot of LLVM's optimization power.
>
> - David
>
>>
>>
>> --
>> Best Regards!
>> Eric Lew
>>
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