-Oz is not respected by LSR (-enable-iv-rewrite revealed fluctuations in code size)

[jsonn]

Bugzilla Link	12342
Version	trunk
OS	Linux
Attachments	Files that compile to different sizes and list
CC	@asl,@atrick,@efriedma-quic

Extended Description

Attached is the list of all files from NetBSD's libkern, libsa and libi386 that differ in code size between -enable-iv-rewrite=false and -enable-iv-rewrite=true. Some of them are not used in this context. The false.txt / true.txt list the object sizes as I see it during the release build. compile.sh /.i will compile the files with both options. Big regressions are e.g. qsort.i, improvements are seen for subr_prf.i

[jsonn]

Good enough. Working around the related PR 12348 side steps the issue completely for me, so keeping the example for analysis is good enough.

[atrick]

There is a lot of luck involved in getting the "right" codegen for a particular loop nest. Performance results are just as jittery as the size results shown here. The good news is:

• the new default looks like a significant improvement in overall size

• with the new default (-enable-iv-rewrite=false) someone writing their own boot loader could conceivably hand tune loops without llvm undoing the work. If llvm still gets in the way, try -mllvm -disable-lsr (as an unsupported workaround).

I can leave this open because there is an opportunity to make the LSR pass aware of the -Oz option, and this is potentially a good example to motivate it. That said, it's very low on my list of enhancements to add.

[jsonn]

Hm. I thought qsort.i had a large regression, but I might have messed up the list at that point. subr_prf.i is what triggered the investigation: it ended up with 120 Bytes overhead.

[atrick]

I'm having trouble knowing what to look at here. When I run compile.sh and simply sum the .o sizes, I get a larger size for enable-iv-rewrite=true. I'm using svn r153254.

false: 54760 bytes true: 55508 bytes

• .o sizes 2596 cd9660.i.false 2676 cd9660.i.true 324 dkcksum.i.false 324 dkcksum.i.true 5032 dosfs.i.false 5052 dosfs.i.true 1948 exec.i.false 1968 exec.i.true 4268 ext2fs.i.false 4264 ext2fs.i.true 3512 ffsv1.i.false 3504 ffsv1.i.true 4156 ffsv2.i.false 4148 ffsv2.i.true 592 gets.i.false 624 gets.i.true 480 ip_cksum.i.false 484 ip_cksum.i.true 3560 lfsv1.i.false 3552 lfsv1.i.true 3592 lfsv2.i.false 3584 lfsv2.i.true 5200 loadfile_elf32.i.false 4684 loadfile_elf32.i.true 4888 loadfile_elf64.i.false 5488 loadfile_elf64.i.true 3596 minixfs3.i.false 3588 minixfs3.i.true 2248 netif.i.false 2248 netif.i.true 2128 qsort.i.false 2780 qsort.i.true 1336 strerror.i.false 1348 strerror.i.true 2024 subr_prf.i.false 1920 subr_prf.i.true 3280 ufs.i.false 3272 ufs.i.true