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Re: [PATCH] Set default to -fomit-frame-pointer
Jeff Law wrote:
> I'd actually prefer to deprecate the H8 and M68k. But assuming that's
> not going to happen in the immediate future I think dropping frame
> pointers on those targets is appropriate as long as we're generating
> dwarf frame info.
Is there a way to check a target does not generate dwarf? I thought that
was the default.
> > I believe in those cases targets already force the frame pointer as required,
> > for example msp430 sets the frame pointer if unwind tables are emitted
> > irrespectively of the command-line or default setting. Various other targets
> > don't even use frame_pointer_needed and just do their own thing.
> I've had the "pleasure" of going round and round on this repeatedly
> through the years with the kernel teams on this. Essentially they
> didn't want to embed the dwarf2 unwinder in the kernel or have all those
> pages of unwind data. Instead they strongly preferred to have a frame
> pointer to facilitate easy and fast unwinding.
A frame pointer does not facilitate unwinding, it can give a backtrace at best.
But indeed, frame pointers, unwinding and stack chains are often confused
despite being completely orthogonal concepts...
> So my concern is to make sure the kernel folks, particularly in the
> ia32 world aren't going to get hosed by this change. If we're changing
> the default it needs to be signaled to them so that they can ensure that
> if they want frame pointers that they still get them.
x86/x64 is not affected since it already omits the frame pointer by default (like
almost all targets in a target specific way). This patch is about making that
the global default precisely because pretty much every target already has it
as the default.
Wilco