Hi everyone,
I don't know if this is an official term for this, but call it "branch
stitching". It's an idea that came to me recently and which I'm trying
out with some success.
The principle is this... I look for blocks of code that look like the
following (using x86) to start:
jmp .L2
.L1:
...
jmp .L3
The code between ".L1" and "jmp .L3" is essentially a self-contained
subroutine since it can only be entered via a jump (the check for a JMP
or RET instruction before .L1 is important to determine it can't enter
the block via normal program flow. After identifying these blocks, I
see if I can 'detach' them from the assembly and reattach them
immediately after a "jmp .L1" instruction, thereby eliminating that jump
and potentially opening up other peephole optimisations - for example,
in the pexpr unit - before:
...
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.
Kit
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