Finding out what backend instruction pattern matches instruction

[Andrew Hutchinson]

I am working on AVR port  and seek advice of the best way working out 
what instructions patterns have been natched to RTL.

This requires adjustment of instruction length to assist branching - 
when operands are finally known. Before this, worst case lengths are 
used from pattern length attributes.

At present, the ADJUST_INSN_LENGTH routine looks at the instruction RTL 
to figure out what pattern was matched, then calls the appropriate 
routine that can do the precise length calculation. The problem with 
this method is that this re-matching can easily be wrong. Great care is 
taken when additional backend patterns are used - or existing ones are 
re-arranged, or  instruction length are calculated incorrectly.

To get around this problem, I replaced this RTL checking with a simple 
lookup of the instruction name using

name = get_insn_name (INSN_CODE (insn));

Then a simple string compare can be used to determine precisely what has 
been matched.

It works fine, but is this an acceptable method ?

The alternative, perhaps,  would be to set each length attribute 
dynamically in each pattern - if that was possible. But that looks like 
way more work.