On 7/17/24 11:13 AM, Georg-Johann Lay wrote:
Am 17.07.24 um 17:55 schrieb Jeff Law:
On 7/17/24 9:26 AM, Georg-Johann Lay wrote:
It looks fine for the trunk. Out of curiosity, does the avr port
implement linker relaxing for this case? That would seem to be
No. avr-ld performs relaxing, but only the two cases of
- JMP/CALL to RJMP/RCALL provided the offset fits.
- [R]CALL+RET to [R]JMP provided there's no label between.
Yea, the first could be comparable to other targets. The second is
probably not all that common since the compiler should be doing that
tail call elimination.
It should. But there are cases where gcc doesn't optimize, like
float add (float a, float b)
{
return a + b;
}
Presumably the a+b is handled via a libcall rather than a normal call?
I guess there might be something in the path where that needs special
handling. It's been like 20+ years since I was last in that code.
Conceptually I don't see a reason why libcalls would need to be special.
Then there are the calls that are not visible to the compiler, like
long mul (long a, long b)
{
return b * a;
}
so that the linker relaxations still have something to do.
Yea, if you're emitting the call behind the back of the compiler for
this kind of case, then the linker is your only real shot. I did
something like that for a few key operations on the mn102 chip eons ago.
One job for Binutils could be optimizing fixed registers like in
char mul3 (char a, char b, char c)
{
return a * b * c;
}
mul3:
mul r22,r20 ; 21 [c=12 l=3] *mulqi3_enh
mov r22,r0
clr r1
mul r22,r24 ; 22 [c=12 l=3] *mulqi3_enh
mov r24,r0
clr r1
ret ; 25 [c=0 l=1] return
The first "clr r1" is void due to the following mul.
Just like GCC PR20296, the only feasible solution is by letting Binutils
do the job. But I have no idea how to adjust branches without labels
like RJMP .+20 that cross an instruction that's optimized out.
I suspect the most important step is to prevent the assembler from
resolving pc-relative jumps and instead emit a suitable relocation.
Once that's done I think the branches should get adjusted automatically.
jeff
Johann
Linker relaxing would be ADD.lo8 + ADC.hi8 => ADD.hi8 which affects
condition code.
In which case it'd only be safe if you knew that CC died before being
used.
jeff
