[cfe-dev] time of inline assembler evaluation in template specialization

[Douglas Gregor]

On Jun 23, 2011, at 12:19 PM, Titus von Boxberg wrote:

> 
> Am 23.06.2011 um 18:45 schrieb John McCall:
> 
>> On Jun 22, 2011, at 11:52 PM, Titus von Boxberg wrote:
>>> I tried to use template specialization for selecting
>>> the right inline assembly code.
>>> 
>>> Example:
>>> -----------------
>>> template<int CPU>
>>> struct A {
>>> };
>>> 
>>> template<>
>>> struct A<8> {
>>> void f(void) {
>>>  asm("ldr r4,=1\n\t" ::: "r4");
>>> }
>>> };
>>> 
>>> template<>
>>> struct A<1> {
>>> void f(void) {
>>>  asm("movl $1, %%eax\n\t" ::: "eax");
>>> }
>>> };
>>> 
>>> 
>>> int main(void)
>>> {
>>> A<1>	a;
>>> a.f();
>>> }
>>> ---------------
>>> clang says:
>>> 
>>> error: unknown register name 'r4' in asm
>>> asm("ldr r4,=1\n\t" ::: "r4");
>>> 
>>> g++ compiles the code, both for arm (A<8>) and x86.
>>> 
>>> Even if the register name r4 in the clobber list would really
>>> be invalid, I'd expect that the compiler would only analyze
>>> the syntactical correctness of the asm statement itself while
>>> parsing the template.
>>> Otherwise this application of template specialization cannot work.
>>> 
>>> Is this assumption correct?
>> 
>> No.  A program is ill-formed, no diagnostic required, if it contains
>> a template specialization with no valid instantiations.  You cannot
>> rely on (non-dependent) code in dead template specializations
>> to not be checked for validity.
> 
> Clear, but elegantly bypassing my question ;-)
> What is - in this context - validity, then?
> Why is it not the mere syntactical correctness of the asm statement?
> asm statements are a nonstandard extension, anyway; so clang is free
> to do it this or the other way.
> If clang would handle it like gcc the combination of template specialization
> and assembly language would work; and I'd recommend to follow gcc
> here since there is no other standard.
> 
> Since you cut away my second question:
> clang already behaves inconsistently (from the user perspective)
> because it analyzes the register lists but not the statement itself.
> That this might be (don't know if it can be with clang) explained
> by the different stages of the compiler / assembler is no excuse.
> 
> And this behaviour is also present when the assembly code is dependent on
> some type parameter.
> 
> What do you think?


Diagnosing ill-formed code within a template definition is a quality-of-implementation issue. Clang's philosophy is to diagnose everything that is allowed and reasonable. It's certainly reasonable to check the constraints, but our architecture makes it a bit hard to check the actual statement. That's a QoI issue that could conceivably be fixed in the future.

Besides, in the example you give, we're not even talking about a template definition: this is actual code that presumably only works in GCC because one of the inline functions isn't ever emitted. I don't think it makes any sense whatsoever to accept this code.

	- Doug