[LLVMdev] LLVM ARM VMLA instruction

[Tim Northover]

> As per Renato comment above, vmla instruction is NEON instruction while vmfa is VFP instruction. Correct me if i am wrong on this.

My version of the ARM architecture reference manual (v7 A & R) lists
versions requiring NEON and versions requiring VFP. (Section
A8.8.337). Split in just the way you'd expect (SIMD variants need
NEON).

> It may seem that total number of cycles are more or less same for single vmla
> and vmul+vadd. However, when vmul+vadd combination is used instead of vmla,
> then intermediate results will be generated which needs to be stored in memory
> for future access.

Well, it increases register pressure slightly I suppose, but there's
no need to store anything to memory unless that gets critical.

> Correct me if i am wrong on this, but my observation till date have shown this.

Perhaps. Actual data is needed, I think, if you seriously want to
change this behaviour in LLVM. The test-suite might be a good place to
start, though it'll give an incomplete picture without the externals
(SPEC & other things).

Of course, if we're just speculating we can carry on.

Cheers.

Tim.