[LLVMdev] PROPOSAL: IR representation of detailed struct assignment information

[Chris Lattner]

On Aug 30, 2012, at 1:34 PM, Krzysztof Parzyszek <kparzysz at codeaurora.org> wrote:
> On 8/22/2012 3:15 PM, Dan Gohman wrote:
>> Here's an example showing the basic problem:
>> 
>> struct bar {
>>   char x;
>>   float y;
>>   double z;
>> };
>> void copy_bar(struct bar *a, struct bar *b) {
>>   *a = *b;
>> }
>> 
>> We get this IR:
>> 
>>   call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* %1, i64 16, i32 8, i1 false)
>> 
>> This works, but it doesn't retain the information that the bytes between fields
>> x and y don't really need to be copied, and it doesn't inform the optimizer
>> that there are three fields with TBAA-relevant types being copied.
>> 
>> The solution I propose here is to have front-ends describe the copy using
>> metadata. For example:
> 
> Why not simply have something like this?
> 
> %1 = load bar* %b
> store %1, bar* %a

That preserves an IR type, but not source level types.  IR types are insufficient for TBAA or "hole" information.

-Chris