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

Dave Martin
2019-06-27T14:38:36Z

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