Re: LLVM 16 (opaque pointers)

From: Dmitry Dolgov <9erthalion6(at)gmail(dot)com>
To: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: LLVM 16 (opaque pointers)
Date: 2023-06-04 09:33:42
Message-ID: 20230604093342.62lqnzy3edj6kwy2@erthalion.local
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> On Mon, May 22, 2023 at 03:38:44PM +1200, Thomas Munro wrote:
> Further changes are already needed for their "main" branch (LLVM
> 17-to-be), so this won't quite be enough to shut seawasp up. At a
> glance, we will need to change from the "old pass manager" API that
> has recently been vaporised[1]
> (llvm-c/Transforms/PassManagerBuilder.h) to the new one[2][3]
> (llvm-c/Transforms/PassBuilder.h), which I suspect/hope will be as
> simple as changing llvmjit.c to call LLVMRunPasses() with a string
> describing the passes we want in "opt -passes" format, instead of our
> code that calls LLVMAddFunctionInlingPass() etc. But that'll be a
> topic for another day, and another thread.
> [1]
> [2]
> [3]

Thanks for tackling the topic! I've tested it with a couple of versions,
LLVM 12 that comes with my Gentoo box, LLVM 15 build from sources and
the modified version of patch adopted for LLVM 17 (build form sources as
well). In all three cases everything seems to be working fine.

Simple benchmarking with a query stolen from some other jit thread
(pgbench running single client with multiple unions of selects a-la
SELECT a, count(*), sum(b) FROM test WHERE c = 2 GROUP BY a) show some
slight performance differences, but nothing dramatic so far. LLVM 17
version produces the lowest latency, with faster generation, inlining
and optimization, but slower emission time. LLVM 12 version produces the
largest latencies with everything except emission timings being slower.
LLVM 15 is somewhere in between.

I'll continue reviewing and, for the records, attach adjustments I was
using for LLVM 17 (purely for testing, not taking into account other
versions), in case if I've missed something.

Attachment Content-Type Size
0002-LLVM-17.patch text/x-diff 4.5 KB

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