Question about insn lengths

[Pontus Lidman <pontus at lysator dot liu dot se>]

Hello,

I'm wondering: how, when and where can I find out the exact length of
an insn?

I'm working on support for the Toshiba MIPS R5900. It has an errata
related to short loops ; loops must not be less than 6 instructions in
length. One solution is to insert nops until the loop is long
enough. To determine the amount of nops, I need to know the exact
number of instruction words that will be generated from a given RTL
insn.

I have a loop lengthening pass based on patches for earlier gcc
versions. I adapted it and inserted it after branch shortening, and I
re-run branch shortening afterwards. This works for most, but not all
cases. The problem seems to be that the instruction lengths returned
by get_attr_length() can sometimes overestimate the length of an insn.

For example, I noticed that the 'mfhilo_si' insn has length attribute
12 (because of possible future nops due to hazards I gather), but
generates only one instruction word (mflo, for example), so the value
I need for this insn is 4.

Any advice is greatly appreciated.

Pontus

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Pontus Lidman, pontus@lysator.liu.se, Software Engineer
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