Re: inefficient code output?

[James E Wilson <wilson at specifixinc dot com>]

Stefan Strasser wrote:
> movl  %ebx, -200(%ebp)
> movl  %ebx, -196(%ebp)
> movl  %eax, 4(%esp)
> movl  -200(%ebp), %edx
> movl  -196(%ebp), %ecx

It is hard to say without a testcase, but my first guess would be reload 
inheritance and/or the post-reload cse pass.

Reload may need to load/store something from a stack slot to satisfy a 
reload.  If we have two instructions in a row that need the same reload, 
then reload inheritance will try to reuse the value.  This may avoid 
loading it twice and/or storing then reloading a value.  If reload 
inheritance is successful, it then tries to delete the now unnecessary 
load/store, but this is a complicated operation, and doesn't always 
succeed.  So sometimes we end up with unnecessary loads/stores for 
partially deleted reloads.

A similar thing might happen if post-reload cse succeeds in simplifying 
some code, but doesn't succeed in deleting the now unnecessary stack 
load/store.  Deleting a stack slot store is difficult, as it requires 
memory alias analysis that probably isn't being done in reload and the 
post-reload cse passes.  It is better to avoid creating them in the 
first place, which is why we have reload inheritance, and why reload is 
so complicated.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com