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Re: RTX costs
- From: Joern Rennecke <joern dot rennecke at embecosm dot com>
- To: "Paulo J. Matos" <pocmatos at gmail dot com>
- Cc: gcc at gcc dot gnu dot org
- Date: Tue, 09 Feb 2010 09:12:25 -0500
- Subject: Re: RTX costs
- References: <4B71694E.70707@gmail.com>
Quoting "Paulo J. Matos" <pocmatos@gmail.com>:
Hi all,
After reading the internal docs about rtx_costs I am left wondering
what they exactly are estimating.
- Are they estimating in the beginning of expand how many insns will be
generated from a particular insn until the assembler is generated?
- or Are they estimating how many assembler instructions will be
generated for a particular insn?
- or something else?
When optimizing for speed, they estimate execution cycles, when
optimizing for space, they estimate code size. In each case,
normalized so that COSTS_N_INSNS (1) is equivalent to the cost of
a simple instruction. Or at least that's the theoretical goal.
Individual ports might deviate from that for historical and/or
practical reasons.