[LLVMdev] [cfe-dev] [PROPOSAL] per-function optimization level control

[Andrea_DiBiagio at sn.scee.net]

Hi,

> Especially since we have support for per function code gen attributes 
now.

I think having function attributes would certainly be useful.
GCC for example provide support for both pragma and function attributes to 
control per-function optimizations. Also, the effect of using pragma 
optimize to control optimizations on a per-function basis is equivalent in 
GCC to specifying the 'optimize' function attribute (followed by a string 
describing an optimization option) for that function.

In my opinion we could also have both pragma and function attributes: 
having a pragma does not mean that we cannot have function attributes.

A very common pattern that we see in our customers' code is a unity build. 
 That is, in order to reduce debug data overhead, and to improve link-time 
they'll group large numbers of their source files together into single 
"unity files".  e.g.

// ------------------
// unity01.cpp
#include <physics.cpp>
#include <textures.cpp>
#include <renderer.cpp>
etc.
// ------------------

In some cases if they've narrowed down a problem, they'll want to debug 
individual functions inside one of these files in which case an attribute 
may be enough. 
However, if they just know, for example, that they have a problem 
somewhere in their texture code they'll often want to do something like 
this:

// ------------------
// unity01.cpp
#include <physics.cpp>

#pragma clang optimize push
#pragma clang optimize "O0" 
#include <textures.cpp>
#pragma clang optimize pop 

#include <renderer.cpp>
etc.
// ------------------

Thanks,
-- Andrea DiBiagio


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