Dimitry Andric via llvm-dev
2017-Feb-05 13:05 UTC
[llvm-dev] clang/llvm support for %= in inline assembly
I encountered this issue of unique asm label names just now, in
https://bugs.freebsd.org/216771 . A very simple test case is this:
void foo(void)
{
asm volatile(
"named_label:\n"
"jnz named_label\n"
);
}
void bar(void)
{
foo();
}
Compiling this with any form of optimization gives an error, because clang is
attempting to inline foo() into bar(), thereby cloning the inline asm part:
$ clang -O -c test.c
test.c:4:5: error: invalid symbol redefinition
"named_label:\n"
^
<inline asm>:1:2: note: instantiated into assembly here
named_label:
^
1 error generated.
This error is very confusing to the end user, IMHO. Either clang should
uniq-ify each of these labels automatically, when inlining functions, or it
should give you advice on how to use anonymous asm labels, or %= macros.
Appending %= to the label name only works if you append at least one ':'
to the asm statement, otherwise it isn't seen as a so-called "assembler
template", apparently:
void foo(void)
{
asm volatile(
"named_label_%=:\n"
"jnz named_label_%=\n"
:
);
}
void bar(void)
{
foo();
}
Resulting in:
$ clang -O -S test.c -o -
.text
.file "test.c"
.globl foo
.p2align 4, 0x90
.type foo, at function
foo: # @foo
# BB#0: # %entry
pushl %ebp
movl %esp, %ebp
#APP
named_label_0:
jne named_label_0
#NO_APP
popl %ebp
retl
.Lfunc_end0:
.size foo, .Lfunc_end0-foo
.globl bar
.p2align 4, 0x90
.type bar, at function
bar: # @bar
# BB#0: # %entry
pushl %ebp
movl %esp, %ebp
#APP
named_label_1:
jne named_label_1
#NO_APP
popl %ebp
retl
.Lfunc_end1:
.size bar, .Lfunc_end1-bar
This is with clang 4.0.0 from the release_40 branch. I have no idea when the %=
support was added.
-Dimitry
> On 05 Feb 2017, at 10:50, Sean Silva via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
>
> I think we may already support it. At least that's the implication from
the post-commit thread of r288093, in particular this reply from Reid
https://www.mail-archive.com/cfe-commits at lists.llvm.org/msg43592.html
>
> Do you have a test case?
>
> -- Sean Silva
>
> On Sat, Feb 4, 2017 at 10:30 PM, Andrew Kelley via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> from https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html
>
> Under certain circumstances, GCC may duplicate (or remove duplicates of)
your assembly code when optimizing. This can lead to unexpected duplicate symbol
errors during compilation if your asm code defines symbols or labels. Using ‘%=’
(see AssemblerTemplate) may help resolve this problem.
>
> ‘%=’
> Outputs a number that is unique to each instance of the asm statement in
the entire compilation. This option is useful when creating local labels and
referring to them multiple times in a single template that generates multiple
assembler instructions.
>
>
> I tried to implement %= in my front-end but I ran into this problem with
the optimizer. So in order to have this feature, LLVM would need to support it
in the IR language.
>
> As far as I can tell, Clang does not support this feature. Is it a
reasonable feature request?
>
> Regards,
> Andrew
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 194 bytes
Desc: Message signed with OpenPGP using GPGMail
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170205/8f1e2a6b/attachment.sig>
Andrew Kelley via llvm-dev
2017-Feb-05 16:19 UTC
[llvm-dev] clang/llvm support for %= in inline assembly
On Sun, Feb 5, 2017 at 8:05 AM, Dimitry Andric <dimitry at andric.com> wrote:> Appending %= to the label name only works if you append at least one ':' > to the asm statement, otherwise it isn't seen as a so-called "assembler > template", apparently:Aha, ok, perfect, this results in LLVM IR: tail call void asm sideeffect "named_label_${:uid}:\0Ajnz named_label_${:uid}\0A", "~{dirflag},~{fpsr},~{flags}"() #2, !dbg !20, !srcloc !25 So LLVM IR does in fact already support it, and my use case is solved. However, it is undocumented in http://llvm.org/docs/LangRef.html. Might be night to add there for future developers. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170205/0d552db0/attachment.html>
Reid Kleckner via llvm-dev
2017-Feb-06 18:20 UTC
[llvm-dev] clang/llvm support for %= in inline assembly
I documented this in r294204. On Sun, Feb 5, 2017 at 8:19 AM, Andrew Kelley via llvm-dev < llvm-dev at lists.llvm.org> wrote:> On Sun, Feb 5, 2017 at 8:05 AM, Dimitry Andric <dimitry at andric.com> wrote: > >> Appending %= to the label name only works if you append at least one ':' >> to the asm statement, otherwise it isn't seen as a so-called "assembler >> template", apparently: > > > Aha, ok, perfect, this results in LLVM IR: > > tail call void asm sideeffect "named_label_${:uid}:\0Ajnz > named_label_${:uid}\0A", "~{dirflag},~{fpsr},~{flags}"() #2, !dbg !20, > !srcloc !25 > > So LLVM IR does in fact already support it, and my use case is solved. > However, it is undocumented in http://llvm.org/docs/LangRef.html. Might > be night to add there for future developers. > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170206/dd6a33f7/attachment.html>