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fuse-caller-save - hook format


All patches for the fuse-caller-save optimization have been ok-ed. The only part
not approved is the MIPS-specific part.

The objection of Richard S. is not so much the patch itself, but more the idea
of the hook fn_other_hard_reg_usage.

For clarity, I'm restating the current hook definition here:
+@deftypefn {Target Hook} bool TARGET_FN_OTHER_HARD_REG_USAGE (struct
hard_reg_set_container *@var{regs})
Add any hard registers to @var{regs} that are set or clobbered by a call to the
function.  This hook only needs to add registers that cannot be found by
examination of the final RTL representation of a function.  This hook returns
true if it managed to determine which registers need to be added.  The default
version of this hook returns false.

Richard prefers to, rather than having a hook specifying what registers are
implicitly clobbered, adding those clobbers to CALL_INSN_FUNCTION_USAGE.

I can see these possibilities (and perhaps there are more):

1. We go with Richards proposal: we make each target responsible for adding
these clobbers in CALL_INSN_FUNCTION_USAGE, and use a hook called f.i.
targetm.fuse_caller_save_p or targetm.implicit_call_clobbers_in_fusage_p, to
indicate whether a target has taken care of that, meaning it's safe to do the
fuse-caller-save optimization.

2. A mixed solution: we make each target responsible for specifying which
clobbers need to be added in CALL_INSN_FUNCTION_USAGE, using a hook called f.i.
targetm.call_clobbered_regs, and add generic code to add those clobbers to

3. We stick with the current, approved hook format, and try to convince Richard
to live with it.

Since you are a register allocator maintainer, familiar with the
fuse-caller-save optimization, and have approved the original hook, I would like
to ask you to make a decision on how to proceed from here.

- Tom

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