On Thu, 5 Jun 2014, Sandra Loosemore wrote: > On 06/05/2014 03:50 PM, Richard Sandiford wrote: > > Sandra Loosemore <san...@codesourcery.com> writes: > > > On 06/05/2014 01:39 AM, Richard Biener wrote: > > > > > > > > [snip] > > > > > > > > Ok, we definitely need to preserve that (documented) behavior. I > > > > suppose > > > > it also sets DECL_USER_ALIGN. -falign-functions is probably another > > > > setter of DECL_ALIGN here. > > > > > > > > If we add a target hook that may adjust function alignment then it > > > > has to honor any user set alignment, then -falign-functions and > > > > then it may only increase alignment over the default FUNCTION_BOUNDARY. > > > > > > > > The point to adjust alignment with the hook may still be output time, > > > > but as we figured it can't simply ignore DECL_ALIGN. > > > > > > Hmmmm. The MIPS tweak we want here is to decrease the alignment for > > > certain functions, so I suppose this means we'd have to go about it > > > backwards by making FUNCTION_BOUNDARY 16 rather than 32 (at least for > > > -Os) and then later increasing it back to 32 for all non-microMIPS > > > functions. > > > > > > Richard S., WDYT? Shall I hack up another patch that does it that way, > > > or do you think that's still the wrong way to go about it? > > > > Well, it sounds like the idea is that FUNCTION_BOUNDARY should basically > > be ignored as a DECL_ALIGN. The only cases where DECL_ALIGN should matter > > are those where the user has set the alignment via command-line options > > or attributes. If so, I'd rather just leave it as it is and make the > > hook pick something smaller. > > > > Just to be sure: I'd imagined this working by having varasm.c detect > > when DECL_ALIGN needs to be honoured and having a hook to call when > > DECL_ALIGN is irrelevant. The default version would then assert > > that DECL_ALIGN is FUNCTION_BOUNDARY and would return that alignment. > > The MIPS hook would override it for the special microMIPS case, > > but would call into the default otherwise. > > > > (Hmm, maybe for that level of detail I should just have written a patch. > > Sorry about that...)
Well, the question is what you initialize DECL_ALIGN to initially and how you ensure that docs are followed for aligned attribute (you can't decrease function alignment by it). Adjusting a pre-existing DECL_ALIGN to sth smaller at any point sounds dangerous. So why not go with Sandras idea instead? I'd transition FUNCTION_BOUNDARY to a target hook like unsigned function_boundary (tree decl); and for decl == NULL return the "default" (the default hook implementation would just return MAX (FUNCTION_BOUNDARY, DECL_ALIGN)). We'd call function_boundary (NULL) from tree.c and later from varasm.c query it again, but this time with the decl specified. And yes, you'd lower your default function_boundary. Richard. > I'm willing to write/test a patch once you guys agree on the semantics of the > new hook and are happy with how it would be used to solve this particular > problem....
