linux-kernel - Re: [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation

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Message-ID: <20190627143826.GG2790@e103592.cambridge.arm.com>
Date:   Thu, 27 Jun 2019 15:38:26 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     linux-arch@...r.kernel.org, Shijith Thotton <sthotton@...vell.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Huw Davies <huw@...eweavers.com>,
        Andre Przywara <andre.przywara@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Will Deacon <will.deacon@....com>, linux-mips@...r.kernel.org,
        Ralf Baechle <ralf@...ux-mips.org>,
        linux-kernel@...r.kernel.org, Paul Burton <paul.burton@...s.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kselftest@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Russell King <linux@...linux.org.uk>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v7 04/25] arm64: Substitute gettimeofday with C
 implementation

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