Re: JIT compiling with LLVM v9.0

From: Andres Freund <andres(at)anarazel(dot)de>
To: Merlin Moncure <mmoncure(at)gmail(dot)com>
Cc: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru>, pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: JIT compiling with LLVM v9.0
Date: 2018-02-09 18:58:35
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On 2018-02-09 09:10:25 -0600, Merlin Moncure wrote:
> Question: when watching the compilation log, I see quite a few files
> being compiled with both O2 and O1, for example:
> clang -Wall -Wmissing-prototypes -Wpointer-arith
> -Wdeclaration-after-statement -Wendif-labels
> -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing
> -fwrapv -Wno-unused-command-line-argument -O2 -O1
> -Wno-ignored-attributes -Wno-unknown-warning-option
> -Wno-ignored-optimization-argument -I../../../../src/include
> -D_GNU_SOURCE -I/home/mmoncure/llvm/include -DLLVM_BUILD_GLOBAL_ISEL
> -D__STDC_LIMIT_MACROS -flto=thin -emit-llvm -c -o nbtsort.bc
> nbtsort.c
> Is this intentional? (didn't check standard compilation, it just jumped out).

It stemms from the following hunk in about emitting
# Add -O1 to the options as clang otherwise will emit 'noinline'
# attributes everywhere, making JIT inlining impossible to test in a
# debugging build.
# FIXME: While LLVM will re-optimize when emitting code (after
# inlining), it'd be better to only do this if -O0 is specified.
%.bc : CFLAGS +=-O1

%.bc : %.c
$(COMPILE.c.bc) -o $@ $<

Inspecting the clang source code it's impossible to stop clang from
emitting noinline attributes for every function on -O0.

I think it makes sense to change this to filtering out -O0 and only
adding -O1 if that's not present. :/


Andres Freund

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