Re: Questions about __attribute((cold))

[John Fine <johnsfine at verizon dot net>]

I'll try some tests to see what that does, but I have two reasons to 
expect that is not what I want.

I would prefer not to put the cold blocks in separate sections, because 
that may require unnecessarily larger branch instructions.  I'm worrying 
primarily about the L1 cache.  For the L1 cache, most of the possible 
benefits are achieved by dropping all the cold blocks right after the 
final return of the function.  There is no need to push them to a 
separate section.  For virtual memory locality, pushing them to a 
separate section would be important.

When I used __builtin_expect to tell the compiler about a cold block, it 
did push that cold block to right after the final return of the 
function.  So without changing any command line option it does know what 
to do with cold blocks.  The question seems to be whether it recognizes 
a block is cold (based on that block calling a cold function).  The 
option you suggest deals with what to do with a cold block not with how 
to recognize a cold block.

Brian Dessent wrote:

> > @item -freorder-blocks-and-partition
> > @opindex freorder-blocks-and-partition
> > In addition to reordering basic blocks in the compiled function, in order
> > to reduce number of taken branches, partitions hot and cold basic blocks
> > into separate sections of the assembly and .o files, to improve
> > paging and cache locality performance.
> >
> >