Re: LLVM 22

On Mon Jan 5, 2026 at 8:50 PM -03, Thomas Munro wrote:
>> > Here's a potential minimal fix. I haven't yet proven that the
>> > optimisation is still working as expected. Probably need to compile
>> > an expression that calls an inlined function and then a non-inlined
>> > function with jit_dump_bitcode=true, then find the right XXX.bc file
>> > under pgdata, llvm-dis XXX.bc, llc XXX.ll, then visually inspect XXX.s
>> > with enough caffeine to confirm that it's not spilling something (ie
>> > store instructions) where previously it didn't, but I wanted to post
>> > what I had so far to see if anyone has a better idea or an easy way to
>> > test it...
>> >
>> I'm not super familiar with reading assembly code but I tried my best to
>> inspect the LLVM 22 and LLVM 21 outputs and if I understood correctly I
>> think that 0002 is working as expected.
>
> Cool. And as another sanity test, if you comment out the new poison
> code so that we don't try to prevent unwanted spills/stores, can you
> see any?
>
Yes, I've commented the poison code block introduced on 0002 and the
generated assembly code seems more bloated, for example:

LLVM 22 with 0002 and the poison code block commented:
LBB2_8:
mov x25, #15624 ; =0x3d08
movk x25, #7427, lsl #16
movk x25, #1, lsl #32
mov x20, #13805 ; =0x35ed
movk x20, #7427, lsl #16
movk x20, #1, lsl #32
ldr x8, [x23]
ldrb w9, [x24]
stur x8, [x25, #-248]
sturb w9, [x25, #-240]
mov w26, #1 ; =0x1
strb w26, [x20, #1419]
ldurb w8, [x25, #-240]
cmp w8, #1
b.eq LBB2_11

LLVM 22 with 0002:
LBB2_8:
mov x20, #40824 ; =0x9f78
movk x20, #19456, lsl #16
movk x20, #1, lsl #32
ldr x8, [x23]
ldrb w9, [x24]
str x8, [x20, #152]
strb w9, [x20, #160]
mov w26, #1 ; =0x1
strb w26, [x20]
cmp w9, #1
b.eq LBB2_11

IIUC with the commented code the LLVM compiler added an extra
load ldurb followed by cmp w8, #1. With the patch it performs a
comparison cmp w9, #1 directly using a register it already has.

--
Matheus Alcantara
EDB: https://www.enterprisedb.com