On 28/11/2022 07:22, J. Gareth Moreton via fpc-devel wrote:
...
testb %al,%al
je .Lj733
subb $1,%al
je .Lj734
jmp .Lj732
.balign 16,0x90
.Lj733:
...
jmp .Lj718
.balign 16,0x90
.Lj732:
movl $2019050530,%ecx
call VERBOSE_$$_INTERNALERROR$LONGINT
jmp .Lj718
The block with the internal error can be moved and 'stitched' to the
"jmp .Lj732" instruction.
...
testb %al,%al
je .Lj733
subb $1,%al
je .Lj734
movl $2019050530,%ecx
call VERBOSE_$$_INTERNALERROR$LONGINT
jmp .Lj718
.balign 16,0x90
.Lj733:
...
I'm still working a few things out, since it can move the function
epilogue which makes things harder to read. Currently I'm only moving
blocks where the label only has a single reference, thereby causing a
dead label when it's stitched alongside its corresponding jump. This
avoids problems where the label is referenced in a data block that's
distinct from the assembly and where moving it may cause problems.
Well first of all, you didn't move the balign in front of .Lj732
In the above example, that may be an improvement (most likely) because
if the label really is referred once only (and thereby is also not a
loop) then it may not be beneficial to align it (except maybe if the
user specified a non default align?).
If the label is referred only once, but the whole think is inside a loop
.... it may still be relevant to have the align? (not sure, depends on
how the cpu caches stuff)?
Another thing is, that moving the block can make the other part of the
loop longer (needing more cache). If this branch-to-be-moved is rarely
entered, it may want to be after the final "jmp-to-loop-start" of the
normal branch?
Of course, if the loop is bigger than the block with the branches, and
we did know that the branch is some sort of exception only, then we
would want to move it even further away, to get it out of the loop......
--------------
Btw, .balign N, 0x90 => isn't there an align that uses multibyte nop
(like) instructions? (I posted some pdf to you a while back, iirc it
points that out)
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