Re: [PATCH v6 1/9] ppc64 (le): prepare for -mprofile-kernel

From: Petr Mladek
Date: Thu Feb 04 2016 - 06:04:05 EST

On Thu 2016-02-04 18:31:40, AKASHI Takahiro wrote:
> Jiri, Torsten
>
> Thank you for your explanation.
>
> On 02/03/2016 08:24 PM, Torsten Duwe wrote:
> >On Wed, Feb 03, 2016 at 09:55:11AM +0100, Jiri Kosina wrote:
> >>On Wed, 3 Feb 2016, AKASHI Takahiro wrote:
> >>>those efforts, we are proposing[1] a new *generic* gcc option, -fprolog-add=N.
> >>>This option will insert N nop instructions at the beginning of each function.
> >
> >>The interesting part of the story with ppc64 is that you indeed want to
> >>create the callsite before the *most* of the prologue, but not really :)
> >
> >I was silently assuming that GCC would do this right on ppc64le; add the NOPs
> >right after the TOC load. Or after TOC load and LR save? ...
>
> On arm/arm64, link register must be saved before any function call. So anyhow
> we will have to add something, 3 instructions at the minimum, like:
> save lr
> branch _mcount
> restore lr
> <prologue>
> ...
> <body>
> ...

So, it is similar to PPC that has to handle LR as well.


> >>The part of the prologue where TOC pointer is saved needs to happen before
> >>the fentry/profiling call.
> >
> >Yes, any call, to any profiler/tracer/live patcher is potentially global
> >and needs the _new_ TOC value.

The code below is generated for PPC64LE with -mprofile-kernel using:

$> gcc --version
gcc (SUSE Linux) 6.0.0 20160121 (experimental) [trunk revision 232670]
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


0000000000000050 <cmdline_proc_show>:
50: 00 00 4c 3c addis r2,r12,0
50: R_PPC64_REL16_HA .TOC.
54: 00 00 42 38 addi r2,r2,0
54: R_PPC64_REL16_LO .TOC.+0x4
58: a6 02 08 7c mflr r0
5c: 01 00 00 48 bl 5c <cmdline_proc_show+0xc>
5c: R_PPC64_REL24 _mcount
60: a6 02 08 7c mflr r0
64: 10 00 01 f8 std r0,16(r1)
68: a1 ff 21 f8 stdu r1,-96(r1)
6c: 00 00 22 3d addis r9,r2,0
6c: R_PPC64_TOC16_HA .toc
70: 00 00 82 3c addis r4,r2,0
70: R_PPC64_TOC16_HA .rodata.str1.8
74: 00 00 29 e9 ld r9,0(r9)
74: R_PPC64_TOC16_LO_DS .toc
78: 00 00 84 38 addi r4,r4,0
78: R_PPC64_TOC16_LO .rodata.str1.8
7c: 00 00 a9 e8 ld r5,0(r9)
80: 01 00 00 48 bl 80 <cmdline_proc_show+0x30>
80: R_PPC64_REL24 seq_printf
84: 00 00 00 60 nop
88: 00 00 60 38 li r3,0
8c: 60 00 21 38 addi r1,r1,96
90: 10 00 01 e8 ld r0,16(r1)
94: a6 03 08 7c mtlr r0
98: 20 00 80 4e blr


And the same function compiled using:

$> gcc --version
gcc (SUSE Linux) 4.8.5
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


0000000000000050 <cmdline_proc_show>:
50: 00 00 4c 3c addis r2,r12,0
50: R_PPC64_REL16_HA .TOC.
54: 00 00 42 38 addi r2,r2,0
54: R_PPC64_REL16_LO .TOC.+0x4
58: a6 02 08 7c mflr r0
5c: 10 00 01 f8 std r0,16(r1)
60: 01 00 00 48 bl 60 <cmdline_proc_show+0x10>
60: R_PPC64_REL24 _mcount
64: a6 02 08 7c mflr r0
68: 10 00 01 f8 std r0,16(r1)
6c: a1 ff 21 f8 stdu r1,-96(r1)
70: 00 00 42 3d addis r10,r2,0
70: R_PPC64_TOC16_HA .toc
74: 00 00 82 3c addis r4,r2,0
74: R_PPC64_TOC16_HA .rodata.str1.8
78: 00 00 2a e9 ld r9,0(r10)
78: R_PPC64_TOC16_LO_DS .toc
7c: 00 00 84 38 addi r4,r4,0
7c: R_PPC64_TOC16_LO .rodata.str1.8
80: 00 00 a9 e8 ld r5,0(r9)
84: 01 00 00 48 bl 84 <cmdline_proc_show+0x34>
84: R_PPC64_REL24 seq_printf
88: 00 00 00 60 nop
8c: 00 00 60 38 li r3,0
90: 60 00 21 38 addi r1,r1,96
94: 10 00 01 e8 ld r0,16(r1)
98: a6 03 08 7c mtlr r0
9c: 20 00 80 4e blr


Please, note that are used either 3 or 4 instructions before the
mcount location depending on the compiler version.

Best Regards,
Petr