Date: Thu, 22 Jan 2026 20:27:29 +0000
To: Matheus Alcantara <matheusssilv97@gmail.com>
From: Pierre Ducroquet <p.psql@pinaraf.info>
Cc: "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Subject: Re: [PATCH] llvmjit: always add the simplifycfg pass
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Le jeudi 22 janvier 2026 =C3=A0 8:54 PM, Matheus Alcantara <matheusssilv97@=
gmail.com> a =C3=A9crit=C2=A0:

> Hi,
>=20
> On 07/01/26 12:08, Pierre Ducroquet wrote:
>=20
> > Hi
> >=20
> > While reading the code generated by llvmjit, I realized the number of L=
LVM basic blocks used in tuple deforming was directly visible in the genera=
ted assembly code with the following code:
> > 0x723382b781c1: jmp 0x723382b781c3
> > 0x723382b781c3: jmp 0x723382b781eb
> > 0x723382b781c5: mov -0x20(%rsp),%rax
> > 0x723382b781..: ... .....
> > 0x723382b781e7: mov %cx,(%rax)
> > 0x723382b781ea: ret
> > 0x723382b781eb: jmp 0x723382b781ed
> > 0x723382b781ed: jmp 0x723382b781ef
> > 0x723382b781ef: jmp 0x723382b781f1
> > 0x723382b781f1: jmp 0x723382b781f3
> > 0x723382b781f3: mov -0x30(%rsp),%rax
> > 0x723382b781..: ... ......
> > 0x723382b78208: mov %rcx,(%rax)
> > 0x723382b7820b: jmp 0x723382b781c5
> > That's a lot of useless jumps, and LLVM has a specific pass to get rid =
of these. The attached patch modifies the llvmjit code to always call this =
pass, even below jit_optimize_above_cost.
> >=20
> > On a basic benchmark (a simple select * from table where f =3D 42), thi=
s optimization saved 7ms of runtime while using only 0.1 ms of extra optimi=
zation time.
>=20
>=20
> The patch needs a rebase due to e5d99b4d9ef.
>=20
> You've added the "simplifycfg" only when the "jit_optimize_above_cost"
> is not triggered which will use the default<O0> and mem2reg passes, the
>=20
> default<O3> pass already include "simplifycfg"?
>=20
>=20
> With e5d99b4d9ef being committed, should we add "simplifycfg" when
> PGJIT_INLINE bit is set since it also use the default<O0> and mem2reg
>=20
> passes?

Hi

Thank you, here is a rebased version of the patch.
To answer your questions:
- O3 already includes simplifycfg, so no need to modify O3
- any code generated by our llvmjit provider, esp. tuple deforming, is heav=
ily dependent on simplifycfg, so when O0 is the basis we should always add =
this pass


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