[RFC] Improve Tree-SSA if-conversion - convergence of efforts

[Richard Guenther]

On 9/12/07, trevor_smigiel@playstation.sony.com
<trevor_smigiel@playstation.sony.com> wrote:
> Tehila asked me a while ago to comment based on my experience with the
> RTL if convert pass and the discussions some of us had at the GCC
> summit.  Sorry it took me so long to respond.
>
> The target I care about (Cell SPU) has some things that make an
> aggressive if convert very useful and profitable.  It has conditional
> moves for every mode (there is a single, unified register file), never
> traps on illegal addresses (addresses always wrap to the 256KB address
> space), and branches are expensive (there is no hardware cache).
>
> The main limitation with the RTL if-convert pass is that it only
> recognizes specific patterns.  It is easy to write a complicated if
> statement (just using normal C with &&/||) that would never get
> converted because it ends up with basic blocks that have many in edges
> that if-convert generally doesn't handle.   (In our internal tree we
> modified the RTL pass to handle some cases of multiple in-edges, and can
> handle any number of insns in a basic block.)
>
> I haven't looked at the tree-SSA if-convert code yet, but based on what
> was described to me at the summit it seemed to be taking the same
> approach as the RTL pass.  Recognize certain patterns and convert it.
>
> I would like to see an approach that is able to take an arbitrary flow
> graph without backedges and convert it to straight line code, limited
> only by the cost model and impossible cases (e.g., inline asm).

We now have (yet another) pass that looks at multiple basic blocks
to catch if-conversion opportunities in &&/|| sequences,
tree-ssa-ifcombine.c, which of course also handles certain special
patterns.  There is also some old patches of mine attached to PR22568
to improve RTL ifcvt to handle multi-basic-block conditional move
sequences.

Richard.