From: Dave Martin <Dave.Martin@arm.com>
To: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: linux-arch@vger.kernel.org,
Shijith Thotton <sthotton@marvell.com>,
Peter Collingbourne <pcc@google.com>,
Arnd Bergmann <arnd@arndb.de>, Huw Davies <huw@codeweavers.com>,
Andre Przywara <andre.przywara@arm.com>,
Daniel Lezcano <daniel.lezcano@linaro.org>,
Will Deacon <will.deacon@arm.com>,
linux-mips@vger.kernel.org, Ralf Baechle <ralf@linux-mips.org>,
linux-kernel@vger.kernel.org, Paul Burton <paul.burton@mips.com>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>,
linux-kselftest@vger.kernel.org,
Catalin Marinas <catalin.marinas@arm.com>,
Russell King <linux@armlinux.org.uk>,
Dmitry Safonov <0x7f454c46@gmail.com>,
Mark Salyzyn <salyzyn@android.com>, Shuah Khan <shuah@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation
Date: Thu, 27 Jun 2019 15:38:26 +0100 [thread overview]
Message-ID: <20190627143826.GG2790@e103592.cambridge.arm.com> (raw)
In-Reply-To: <a07b66cb-186f-a743-4f1d-41227f23db74@arm.com>
On Thu, Jun 27, 2019 at 12:59:07PM +0100, Vincenzo Frascino wrote:
> On 6/27/19 12:27 PM, Dave Martin wrote:
> > On Thu, Jun 27, 2019 at 11:57:36AM +0100, Vincenzo Frascino wrote:
[...]
> >> Disassembly of section .text:
> >> 0000000000000000 show_it:
> >> 0: e8 03 1f aa mov x8, xzr
> >> 4: 09 68 68 38 ldrb w9, [x0, x8]
> >> 8: 08 05 00 91 add x8, x8, #1
> >> c: c9 ff ff 34 cbz w9, #-8 <show_it+0x4>
> >> 10: 02 05 00 51 sub w2, w8, #1
> >> 14: e1 03 00 aa mov x1, x0
> >> 18: 08 08 80 d2 mov x8, #64
> >> 1c: 01 00 00 d4 svc #0
> >> 20: c0 03 5f d6 ret
> >>
> >> Commands used:
> >>
> >> $ clang -target aarch64-linux-gnueabi main.c -O -c -o main.clang.<x>.o
> >> $ llvm-objdump -d main.clang.<x>.o
> >
> > Actually, I'm not sure this is comparable with the reproducer I quoted
> > in my last reply.
> >
>
> As explained in my previous email, this is the only case that can realistically
> happen. vDSO has no dependency on any other library (i.e. libgcc you were
> mentioning) and we are referring to the fallbacks which fall in this category.
Outlining could also introduce a local function call where none exists
explicitly in the program IIUC.
My point is that the interaction between asm reg vars and machine-level
procedure calls is at best ill-defined, and it is largely up to the
compiler when to introduce such a call, even without LTO etc.
So we should not be surprised to see variations in behaviour depending
on compiler, compiler version and compiler flags.
> > The compiler can see the definition of strlen and fully inlines it.
> > I only ever saw the problem when the compiler emits an out-of-line
> > implicit function call.
> > > What does clang do with my example on 32-bit?
>
> When clang is selected compat vDSOs are currently disabled on arm64, will be
> introduced with a future patch series.
>
> Anyway since I am curious as well, this is what happens with your example with
> clang.8 target=arm-linux-gnueabihf:
>
> dave-code.clang.8.o: file format ELF32-arm-little
>
> Disassembly of section .text:
> 0000000000000000 foo:
> 0: 00 00 00 ef svc #0
> 4: 1e ff 2f e1 bx lr
>
> 0000000000000008 bar:
> 8: 10 4c 2d e9 push {r4, r10, r11, lr}
> c: 08 b0 8d e2 add r11, sp, #8
> 10: 00 40 a0 e1 mov r4, r0
> 14: fe ff ff eb bl #-8 <bar+0xc>
> 18: 00 10 a0 e1 mov r1, r0
> 1c: 04 00 a0 e1 mov r0, r4
> 20: 00 00 00 ef svc #0
> 24: 10 8c bd e8 pop {r4, r10, r11, pc}
> Compiled with -O2, -O3, -Os never inlines.
Looks sane, and is the behaviour we want.
> Same thing happens for aarch64-linux-gnueabi:
>
> dave-code.clang.8.o: file format ELF64-aarch64-little
>
> Disassembly of section .text:
> 0000000000000000 foo:
> 0: e0 03 00 2a mov w0, w0
> 4: e1 03 01 2a mov w1, w1
> 8: 01 00 00 d4 svc #0
> c: c0 03 5f d6 ret
>
> 0000000000000010 bar:
> 10: 01 0c c1 1a sdiv w1, w0, w1
> 14: e0 03 00 2a mov w0, w0
> 18: 01 00 00 d4 svc #0
> 1c: c0 03 5f d6 ret
Curious, clang seems to be inserting some seemingly redundant moves
of its own here, though this shouldn't break anything.
I suspect that clang might require an X-reg holding an int to have its
top 32 bits zeroed for passing to an asm, whereas GCC does not. I think
this comes under "we should not be surprised to see variations".
GCC 9 does this instead:
0000000000000000 <foo>:
0: d4000001 svc #0x0
4: d65f03c0 ret
0000000000000008 <bar>:
8: 1ac10c01 sdiv w1, w0, w1
c: d4000001 svc #0x0
10: d65f03c0 ret
> Based on this I think we can conclude our investigation.
So we use non-reg vars and use the asm clobber list and explicit moves
to get things into / out of the right registers?
Cheers
---Dave
next prev parent reply other threads:[~2019-06-27 14:38 UTC|newest]
Thread overview: 108+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-21 9:52 [PATCH v7 00/25] Unify vDSOs across more architectures Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 01/25] kernel: Standardize vdso_datapage Vincenzo Frascino
2019-06-24 13:56 ` Catalin Marinas
2019-06-21 9:52 ` [PATCH v7 02/25] kernel: Define gettimeofday vdso common code Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 03/25] kernel: Unify update_vsyscall implementation Vincenzo Frascino
2019-06-21 10:49 ` Huw Davies
2019-06-21 9:52 ` [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation Vincenzo Frascino
2019-06-24 13:36 ` Will Deacon
2019-06-24 13:59 ` Vincenzo Frascino
2019-06-25 16:18 ` [PATCH 1/3] lib/vdso: Delay mask application in do_hres() Vincenzo Frascino
2019-06-25 16:18 ` [PATCH 2/3] arm64: Fix __arch_get_hw_counter() implementation Vincenzo Frascino
2019-06-25 16:18 ` [PATCH 3/3] arm64: compat: " Vincenzo Frascino
2019-06-25 17:02 ` [PATCH 1/3] lib/vdso: Delay mask application in do_hres() Thomas Gleixner
2019-06-25 18:27 ` Thomas Gleixner
2019-06-25 20:15 ` Andy Lutomirski
2019-06-25 22:24 ` Thomas Gleixner
2019-06-26 6:38 ` Thomas Gleixner
2019-06-26 9:25 ` Vincenzo Frascino
2019-06-26 10:02 ` lib/vdso: Make delta calculation work correctly Thomas Gleixner
2019-06-26 11:08 ` Vincenzo Frascino
2019-06-24 13:58 ` [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation Catalin Marinas
2019-06-25 15:33 ` Dave Martin
2019-06-26 13:27 ` Vincenzo Frascino
2019-06-26 16:14 ` Dave Martin
2019-06-26 19:01 ` Vincenzo Frascino
2019-06-27 10:01 ` Dave Martin
2019-06-27 10:57 ` Vincenzo Frascino
2019-06-27 11:27 ` Dave Martin
2019-06-27 11:59 ` Vincenzo Frascino
2019-06-27 14:38 ` Dave Martin [this message]
2019-06-27 15:34 ` Vincenzo Frascino
2019-06-25 17:43 ` [PATCH] arm64: vdso: Fix compilation with clang < 8 Vincenzo Frascino
2019-06-26 11:36 ` [PATCH v2] arm64: vdso: Fix compilation with clang older then 8 Vincenzo Frascino
[not found] ` <CGME20190628130921eucas1p239935b0771032c331911eacc1a69dd2e@eucas1p2.samsung.com>
2019-06-28 13:09 ` [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation Marek Szyprowski
2019-06-28 14:32 ` Vincenzo Frascino
2019-06-28 16:50 ` Sylwester Nawrocki
2019-06-29 6:58 ` Vincenzo Frascino
2019-07-08 12:57 ` Sylwester Nawrocki
2019-07-08 13:09 ` Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 05/25] arm64: Build vDSO with -ffixed-x18 Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 06/25] arm64: compat: Add missing syscall numbers Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 07/25] arm64: compat: Expose signal related structures Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 08/25] arm64: compat: Generate asm offsets for signals Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 09/25] lib: vdso: Add compat support Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 10/25] arm64: compat: Add vDSO Vincenzo Frascino
2019-06-24 14:00 ` Catalin Marinas
2019-07-10 4:02 ` John Stultz
2019-07-10 6:12 ` Thomas Gleixner
2019-07-10 9:48 ` Vincenzo Frascino
2019-07-10 8:27 ` Will Deacon
2019-07-10 8:58 ` Thomas Gleixner
2019-07-10 9:12 ` Will Deacon
2019-07-10 9:47 ` Vincenzo Frascino
2019-07-10 13:41 ` Vincenzo Frascino
2019-07-10 13:04 ` [PATCH] arm64: vdso: Fix ABI regression in compat vdso Vincenzo Frascino
2019-07-10 13:25 ` Will Deacon
2019-07-10 13:42 ` Vincenzo Frascino
2019-07-10 14:01 ` [PATCH v2] " Vincenzo Frascino
2019-07-10 15:44 ` John Stultz
2019-07-10 15:53 ` Vincenzo Frascino
2019-07-11 9:45 ` Will Deacon
2019-07-11 10:34 ` Thomas Gleixner
2019-07-11 11:32 ` Will Deacon
2019-06-21 9:52 ` [PATCH v7 11/25] arm64: Refactor vDSO code Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 12/25] arm64: compat: vDSO setup for compat layer Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 13/25] arm64: elf: vDSO code page discovery Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 14/25] arm64: compat: Get sigreturn trampolines from vDSO Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 15/25] arm64: Add vDSO compat support Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 16/25] arm: Add support for generic vDSO Vincenzo Frascino
2019-12-04 13:51 ` [PATCH v7 16/25] arm: Add support for generic vDSO (causing crash) Guenter Roeck
2019-12-04 13:58 ` Vincenzo Frascino
2019-12-04 16:16 ` Guenter Roeck
2019-12-04 17:15 ` Vincenzo Frascino
2019-12-04 19:39 ` Guenter Roeck
2019-12-05 9:42 ` Philippe Mathieu-Daudé
2019-12-05 10:00 ` Vincenzo Frascino
2019-12-05 11:02 ` Arnd Bergmann
2019-12-05 14:56 ` Philippe Mathieu-Daudé
2019-06-21 9:52 ` [PATCH v7 17/25] arm: Add clock_getres entry point Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 18/25] arm: Add clock_gettime64 " Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 19/25] mips: Add support for generic vDSO Vincenzo Frascino
2019-07-26 5:15 ` Paul Burton
2019-07-26 16:29 ` [PATCH 0/2] mips: vdso: Fix Makefile Vincenzo Frascino
2019-07-26 16:29 ` [PATCH 1/2] mips: vdso: Fix source path Vincenzo Frascino
2019-07-26 16:29 ` [PATCH 2/2] mips: vdso: Fix flip/flop vdso building bug Vincenzo Frascino
2019-07-28 22:20 ` [PATCH 0/2] mips: vdso: Fix Makefile Paul Burton
2019-06-21 9:52 ` [PATCH v7 20/25] mips: Add clock_getres entry point Vincenzo Frascino
2019-07-26 5:15 ` Paul Burton
2019-06-21 9:52 ` [PATCH v7 21/25] mips: Add clock_gettime64 " Vincenzo Frascino
2019-07-26 5:15 ` Paul Burton
2019-06-21 9:52 ` [PATCH v7 22/25] x86: Add support for generic vDSO Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 23/25] x86: Add clock_getres entry point Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 24/25] x86: Add clock_gettime64 " Vincenzo Frascino
2019-06-21 9:52 ` [PATCH v7 25/25] kselftest: Extend vDSO selftest Vincenzo Frascino
2019-06-24 0:34 ` [PATCH v7 00/25] Unify vDSOs across more architectures Thomas Gleixner
2019-06-24 1:15 ` Andy Lutomirski
2019-06-24 7:42 ` Thomas Gleixner
2019-06-24 13:21 ` Vincenzo Frascino
2019-06-24 14:18 ` Thomas Gleixner
2019-06-24 14:23 ` Russell King - ARM Linux admin
2019-06-24 14:49 ` Catalin Marinas
2019-06-24 16:20 ` Vincenzo Frascino
2019-10-25 11:42 ` Geert Uytterhoeven
2019-06-24 18:41 ` Paul Burton
2019-06-24 23:16 ` Vincenzo Frascino
2019-06-25 17:11 ` Paul Burton
2019-06-25 17:17 ` Vincenzo Frascino
2019-06-24 12:50 ` Andre Przywara
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