On 12/21/2009 01:13 AM, Laurent Desnogues wrote:
The question for the generalized movcond is how useful is it?
Which front-ends would need it and would the cost to generate
code for it on some (most?) back-ends be amortized?
... Any front end that has a conditional move instruction?
Sparcv9, Mips32, Alpha, ARM...
That said, I think the *biggest* gains are to be had because with
movcond -- at least on some targets -- we can have one BB per TB, and
avoid any intermediate spilling of global registers back to memory.
My guess (I use that word given that I didn't do any benchmark
to sustain my claim) is that your implementation is too complex.
Too complex for what? The message against which you are quoting has an
implementation of 2 lines.
Of course setcond can be implemented in terms of movcond,
but my guess (again that word...) is that setcond could be
enough and even faster in most cases.
To implement condition codes, yes, to implement compare instructions
(e.g. mips slt, alpha cmp{eq,lt,lte}), yes. To implement conditional
moves, no. At least not without using 5 instructions where 1 would suffice.
Regarding your patches, I would like to see setcond put in
mainline with a simplified version for i386.
Again, simplified from what? The last setcond implementation was 2 lines.
r~
