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

Dave Martin
2019-06-27T11:27:38Z

On Thu, Jun 27, 2019 at 11:57:36AM +0100, Vincenzo Frascino wrote:
> Hi Dave,
>
> Overall, I want to thank you for bringing out the topic. It helped me to
> question some decisions and make sure that we have no holes left in
> the approach.

Fair enough.

This is really just a nasty compiler corner-case... the validity of the
overall approach isn't affected.

> >>
> >> vDSO library is a shared object not compiled with LTO as far as I can
> >> see, hence if this involved LTO should not applicable in this case.
> >
> > That turned to be a spurious hypothesis on my part -- LTO isn't the
> > smoking gun. (See below.)
> >
>
> Ok.
>
> >>> The classic example of this (triggered directly and not due to inlining)
> >>> would be something like:
> >>>
> >>> int bar(int, int);
> >>>
> >>> void foo(int x, int y)
> >>> {
> >>> register int x_ asm("r0") = x;
> >>> register int y_ asm("r1") = bar(x, y);
> >>>
> >>> asm volatile (
> >>> "svc #0"
> >>> :: "r" (x_), "r" (y_)
> >>> : "memory"
> >>> );
> >>> }
> >>>
> >>> ->
> >>>
> >>> 0000000000000000 <foo>:
> >>> 0: a9bf7bfd stp x29, x30, [sp, #-16]!
> >>> 4: 910003fd mov x29, sp
> >>> 8: 94000000 bl 0 <bar>
> >>> c: 2a0003e1 mov w1, w0
> >>> 10: d4000001 svc #0x0
> >>> 14: a8c17bfd ldp x29, x30, [sp], #16
> >>> 18: d65f03c0 ret
> >>>
> >>
> >> Contextualized to what my vdso fallback functions do, this should not be a
> >> concern because in no case a function result is directly set to a variable
> >> declared as register.
> >>
> >> Since the vdso fallback functions serve a very specific and limited purpose, I
> >> do not expect that that code is going to change much in future.
> >>
> >> The only thing that can happen is something similar to what I wrote in my
> >> example, which as I empirically proved does not trigger the problematic behavior.
> >>
> >>>
> >>> The gcc documentation is vague and ambiguous about precisely whan this
> >>> can happen and about how to avoid it.
> >>>
> >>
> >> On this I agree, it is not very clear, but this seems more something to raise
> >> with the gcc folks in order to have a more "explicit" description that leaves no
> >> room to the interpretation.
> >>
> >> ...
> >>
> >>>
> >>> However, the workaround is cheap, and to avoid the chance of subtle
> >>> intermittent code gen bugs it may be worth it:
> >>>
> >>> void foo(int x, int y)
> >>> {
> >>> asm volatile (
> >>> "mov x0, %0\n\t"
> >>> "mov x1, %1\n\t"
> >>> "svc #0"
> >>> :: "r" (x), "r" (bar(x, y))
> >>> : "r0", "r1", "memory"
> >>> );
> >>> }
> >>>
> >>> ->
> >>>
> >>> 0000000000000000 <foo>:
> >>> 0: a9be7bfd stp x29, x30, [sp, #-32]!
> >>> 4: 910003fd mov x29, sp
> >>> 8: f9000bf3 str x19, [sp, #16]
> >>> c: 2a0003f3 mov w19, w0
> >>> 10: 94000000 bl 0 <bar>
> >>> 14: 2a0003e2 mov w2, w0
> >>> 18: aa1303e0 mov x0, x19
> >>> 1c: aa0203e1 mov x1, x2
> >>> 20: d4000001 svc #0x0
> >>> 24: f9400bf3 ldr x19, [sp, #16]
> >>> 28: a8c27bfd ldp x29, x30, [sp], #32
> >>> 2c: d65f03c0 ret
> >>>
> >>>
> >>> What do you think?
> >>>
> >>
> >> The solution seems ok, thanks for providing it, but IMHO I think we
> >> should find a workaround for something that is broken, which, unless
> >> I am missing something major, this seems not the case.
> >
> > So, after a bit of further experimentation, I found that I could trigger
> > it with implicit function calls on an older compiler. I couldn't show
> > it with explicit function calls (as in your example).
> >
> > With the following code, inlining if an expression that causes an
> > implicit call to a libgcc helper can trigger this issue, but I had to
> > try an older compiler:
> >
> > int foo(int x, int y)
> > {
> > register int res asm("r0");
> > register const int x_ asm("r0") = x;
> > register const int y_ asm("r1") = y;
> >
> > asm volatile (
> > "svc #0"
> > : "=r" (res)
> > : "r" (x_), "r" (y_)
> > : "memory"
> > );
> >
> > return res;
> > }
> >
> > int bar(int x, int y)
> > {
> > return foo(x, x / y);
> > }
> >
> > -> (arm-linux-gnueabihf-gcc 9.1 -O2)
> >
> > 00000000 <foo>:
> > 0: df00 svc 0
> > 2: 4770 bx lr
> >
> > 00000004 <bar>:
> > 4: b510 push {r4, lr}
> > 6: 4604 mov r4, r0
> > 8: f7ff fffe bl 0 <__aeabi_idiv>
> > c: 4601 mov r1, r0
> > e: 4620 mov r0, r4
> > 10: df00 svc 0
> > 12: bd10 pop {r4, pc}
> >
> > -> (arm-linux-gnueabihf-gcc 5.1 -O2)
> >
> > 00000000 <foo>:
> > 0: df00 svc 0
> > 2: 4770 bx lr
> >
> > 00000004 <bar>:
> > 4: b508 push {r3, lr}
> > 6: f7ff fffe bl 0 <__aeabi_idiv>
> > a: 4601 mov r1, r0
> > c: df00 svc 0
> > e: bd08 pop {r3, pc}
> >
>
> Thanks for reporting this. I had a go with gcc-5.1 on the vDSO library and seems
> Ok, but it was worth trying.
>
> For obvious reasons I am not reporting the objdump here :)
>
> > I was struggling to find a way to emit an implicit function call for
> > AArch64, except for 128-bit divide, which would complicate things since
> > uint128_t doesn't fit in a single register anyway.
> >
> > Maybe this was considered a bug and fixed sometime after GCC 5, but I
> > think the GCC documentation is still quite unclear on the semantics of
> > register asm vars that alias call-clobbered registers in the PCS.
> >
> > If we can get a promise out of the GCC folks that this will not happen
> > with any future compiler, then maybe we could just require a new enough
> > compiler to be used.
> >
>
> On this I fully agree, the compiler should never change an "expected" behavior.
>
> If the issue comes from a gray area in the documentation, we have to address it
> and have it fixed there.
>
> The minimum version of the compiler from linux-4.19 is 4.6, hence I had to try
> that the vDSO lib does not break with 5.1 [1].
>
> [1]
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cafa0010cd51fb711fdcb50fc55f394c5f167a0a

OK

> > Then of course there is clang.
> >
>
> I could not help myself and I tried clang.8 and clang.7 as well with my example,
> just to make sure that we are fine even in that case. Please find below the
> results (pretty identical).
>
> main.clang.7.o: file format ELF64-aarch64-little
>
> 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
>
> main.clang.8.o: file format ELF64-aarch64-little
>
> 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.

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?

Cheers
---Dave