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

[Dave Martin]

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