Re: Auto-vectorization speeds up multiplication of large-precision numerics

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com>
Cc: Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Auto-vectorization speeds up multiplication of large-precision numerics
Date: 2020-09-07 05:53:22
Message-ID: 1644558.1599458002@sss.pgh.pa.us
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I wrote:
> I made some cosmetic changes to this and committed it.

BTW, poking at this further, it seems that the patch only really
works for gcc. clang accepts the -ftree-vectorize switch, but
looking at the generated asm shows that it does nothing useful.
Which is odd, because clang does do loop vectorization.

I tried adding -Rpass-analysis=loop-vectorize and got

numeric.c:8341:3: remark: loop not vectorized: could not determine number of loop iterations [-Rpass-analysis=loop-vectorize]
for (i2 = 0; i2 <= i; i2++)
^

which is interesting but I don't know how to proceed further.

regards, tom lane

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