GCC Front-End Questions

[Jeff Law]

On 12/08/2015 12:32 PM, Jodi A. Miller wrote:
> Good Afternoon,
>
> My colleagues and I are doing a study on compilers.  We have some
> questions related to gcc and the compilation process that we were
> hoping you could help us with.
>
> In a general sense, we were wondering what optimizations may be
> occurring in the front-end stages of the compiler?  We are noticing
> some constant substitution (e.g., 2*15+3 being changed to 33) and
> some algebraic simplifications (more later) showing up in the first
> intermediate file generated using dump flags with the g++ command.
> Are we correct in assuming this first IR file is indeed coming from
> the front end (.original file)?  In looking at the gcc source code,
> we are having difficulty seeing where these optimizations might be
> occurring in the initial parsing phase.  Do you have any insights you
> might be able to share?
Front-ends do a fair amount of folding, distribution & reassociation 
(see fold-const.c).  We are currently working to delay folding until the 
optimization pipeline starts.

Essentially the guiding principle of that work is that the initial 
gimple code ought to closely match the input source.

The first batch of work to delay folding will show up in gcc-6 for the 
C++ front-end.

jeff