[LLVMdev] Language lawyer question

[Dale Johannesen]

On Mar 11, 2008, at 9:47 PM, Patrick Meredith wrote:

> I thought pointer referencing like this was only valid for arrays.   
> I could be wrong,  but it might be that looping over the struct like  
> that
> is invalid, making it undefined behavior (and then the hole doesn't  
> matter because there is no valid way to access it).  That said, I've  
> definitely
> seen a lot of code that uses pointers to reference struct contents.

Anything can be addressed as characters.  C99 6.5, see last line:

An object shall have its stored value accessed only by an lvalue  
expression that has one of
the following types:73)
—atype compatible with the effective type of the object,
—aqualified version of a type compatible with the effective type of  
the object,
—atype that is the signed or unsigned type corresponding to the  
effective type of the
object,
—atype that is the signed or unsigned type corresponding to a  
qualified version of the
effective type of the object,
—anaggregate or union type that includes one of the aforementioned  
types among its
members (including, recursively,amember of a subaggregate or contained  
union), or
—a character type.

C89 and C++ have similar language.

On Mar 11, 2008, at 10:22 PM, Shantonu Sen wrote:
> Does the test case indicate the why it was added?

Actually it's testing something else entirely and tripped over this  
before it got to what it's supposed to be testing:(
Just assumed it would work, as it does with gcc's codegen, of course.

> More of an implementation observation than a language  
> interpretation, but if you add a "long l[6];" field, llvm-gcc  
> continues to do field-by-field copies, but at l[7] it turns into  
> machine-word copies, then at some point it turns into a rep/movsl  
> (on Intel), and then at another threshold it turns into a memcpy(3)  
> callout.
>
> What part of LLVM's codegen for copying "struct x { char c; short s;  
> long l[6] };" considers a movb + movw + 6 movl's to efficient in  
> either time or space (I was using -Os)? What changes when the  
> overall structure gets to 64 bytes such that it decides its more  
> efficient to copy a word at a time?

Yeah, it's not efficient either.  I didn't want to get into that since  
fixing the correctness issue, if there is one, will automatically fix  
this too.
I think the justification is that breaking the struct into fields  
early makes it easier to do other optimizations.

> I think the test case is bogus in terms of language correctness,

Why?

> but it might be indicative of a missed optimization for doing  
> structure copies. Is that what GCC's test case is actually trying to  
> validate? If so, it probably falls under a "gcc test case" and not a  
> "C test case", if one can differentiate them.

There certainly are "gcc test cases", but so far I don't think this is  
one.

> Maybe it would be reasonable for llvm-gcc to NOT copy the padding at  
> -O0 and do explicit field copies, but to copy the padding as a side  
> effect of an inlined memcpy() implementation for copying  
> sizeof(struct x) when optimization is used. Copying using the  
> largest appropriate registers/instructions given the structure size  
> and alignment seems like it would always be faster than field  
> copies, even for small structures.


> On Mar 11, 2008, at 10:42 PM, Dale Johannesen wrote:
>
>> Looking through the gcc testsuite turned up an interesting edge  
>> case.  Let's assume our target leaves a hole for alignment in  
>> struct x, as do x86 and powerpc.  Do you think the following code  
>> can validly abort?
>>
>>   struct x { char c; short s; };
>>   int i;    char *p;
>>   memset(&X, 0, sizeof(struct x));
>>   memset(&Y, 22, sizeof(struct x));
>>   X = Y;
>>   for (i=0, p=(char *)&X; i<sizeof(struct x); i++, p++)
>>     if (*p != 22)
>>       abort();
>>
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