Date: Tue, 20 Jul 2021 08:57:23 -0700
From: Andres Freund <andres@anarazel.de>
To: David Rowley <dgrowleyml@gmail.com>
Cc: PostgreSQL Development <pgsql-hackers@postgresql.org>,
	Robert Haas <robertmhaas@gmail.com>,
	Michael Paquier <michael.paquier@gmail.com>,
	Tomas Vondra <tv@fuzzy.cz>
Subject: Re: Avoid stack frame setup in performance critical routines using
 tail calls
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Hi,

On 2021-07-20 19:37:46 +1200, David Rowley wrote:
> On Tue, 20 Jul 2021 at 19:04, Andres Freund <andres@anarazel.de> wrote:
> > > * AllocateSetAlloc.txt
> > > * palloc.txt
> > > * percent.txt
> >
> > Huh, that's interesting. You have some control flow enforcement stuff turned on (the endbr64). And it looks like it has a non zero cost (or maybe it's just skid). Did you enable that intentionally? If not, what compiler/version/distro is it? I think at least on GCC that's -fcf-protection=...
>
> It's ubuntu 21.04 with gcc 10.3 (specifically gcc version 10.3.0
> (Ubuntu 10.3.0-1ubuntu1)
>
> I've attached the same results from compiling with clang 12
> (12.0.0-3ubuntu1~21.04.1)

It looks like the ubuntu folks have changed the default for CET to on.


andres@ubuntu2020:~$ echo 'int foo(void) { return 17;}' > test.c && gcc -O2  -c -o test.o test.c && objdump -S test.o

test.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <foo>:
   0:	f3 0f 1e fa          	endbr64
   4:	b8 11 00 00 00       	mov    $0x11,%eax
   9:	c3                   	retq
andres@ubuntu2020:~$ echo 'int foo(void) { return 17;}' > test.c && gcc -O2 -fcf-protection=none -c -o test.o test.c && objdump -S test.o

test.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <foo>:
   0:	b8 11 00 00 00       	mov    $0x11,%eax
   5:	c3                   	retq


Independent of this patch, it might be worth running a benchmark with
the default options, and one with -fcf-protection=none. None of my
machines support it...

$ cpuid -1|grep CET
      CET_SS: CET shadow stack                 = false
      CET_IBT: CET indirect branch tracking    = false
         XCR0 supported: CET_U state          = false
         XCR0 supported: CET_S state          = false

Here it adds about 40kB of .text, but I can't measure the CET
overhead...

Greetings,

Andres Freund