[RFC: Patch 1/6] New target hook: rtx_branch_cost

[James Greenhalgh]

Hi,

This patch introduces a new target hook, to be used like BRANCH_COST but
with a guaranteed unit of measurement. We want this to break away from
the current ambiguous uses of BRANCH_COST.

BRANCH_COST is used in ifcvt.c in two types of comparisons. One against
instruction counts - where it is used as the limit on the number of new
instructions we are permitted to generate. The other (after multiplying
by COSTS_N_INSNS (1)) directly against RTX costs.

Of these, a comparison against RTX costs is the more easily understood
metric across the compiler, and the one I've pulled out to the new hook.
To keep things consistent for targets which don't migrate, this new hook
has a default value of BRANCH_COST * COSTS_N_INSNS (1).

OK?

Thanks,
James

---
2016-06-02  James Greenhalgh  <james.greenhalgh@arm.com>

	* target.def (rtx_branch_cost): New.
	* doc/tm.texi.in (TARGET_RTX_BRANCH_COST): Document it.
	* doc/tm.texi: Regenerate.
	* targhooks.h (default_rtx_branch_cost): New.
	* targhooks.c (default_rtx_branch_cost): New.
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