Re: 0003-Improve-VBEout-computation.patch

[Jeff Law <law at redhat dot com>]

On 06/16/10 09:56, Maxim Kuvyrkov wrote:
> Code hoisting algorithm from Muchnick, which is the one implemented in 
> GCC, has a quirk in that it does include expressions calculated in 
> basic block in its VBEout set.  This seems odd to me; if an expression 
> is calculated in BB and is available at BB's end, then we do want to 
> hoist expressions from BB's successors to the end of BB.  This patch 
> implements this.
>
> The patch also adds an open-ended comment which describes another 
> possible improvement to the algorithm.
I think the comment that has sparked the confusion needs to be removed 
as the proposed optimization is incorrect, at least in cases where the 
expression may trap.


+      /* Enable if debugging VBE data flow problem.  */
+      if (dump_file && 0)
+        {
+          fprintf (dump_file, "vbein (%d): ", bb->index);
+          dump_sbitmap_file (dump_file, hoist_vbein[bb->index]);
+          fprintf (dump_file, "vbeout(%d): ", bb->index);
+          dump_sbitmap_file (dump_file, hoist_vbeout[bb->index]);
+        }

ISTM you should move this code to the end of compute_code_hoist_vbeinout 
and have it iterate over each block -- you don't want to dump the VBE 
state at each transition change, ISTM you want to dump the vbein/vbeout 
for each block when the solution is complete.

Jeff