does the instruction combiner regards (foo & 0xff) as a special case?

[Joern RENNECKE]

>> But I found they fails to match
>> 
>> if(foo & 0xff) and if(foo & 0xffff)

These get simplified to foo.

> Look at the debugging dump before the combine pass to see what you
> need to match.

It doesn't work that way.  What you get from there are only the insn numbers.

Then you run cc1 (or whatever languiage-specific compiler you use) under
gdb control, with a breakpoint on the point in try_combine - in this case,
before the first recog_for_combine call - with a condition to match the
insn numbers.  E.g. for breakpoint 5, to match any combination that ends
in insn 42,
you say:

cond 5 i3->u.fld[0].rt_int == 42

to match a combination of insns 40, 41 and 42 (and only in exactly that
order):

cond 5 
i3->u.fld[0].rt_int == 42 && i2->u.fld[0].rt_int == 41 && i1 && i1->u.fld[0].rt_int == 40

or for an older codebase:

i3->fld[0].rtint == 42 && i2->fld[0].rtint == 41 && i1 && i1->fld[0].rtint == 40