JDK-6510732 : Compute consistent block frequencies
  • Type: Bug
  • Component: hotspot
  • Sub-Component: compiler
  • Affected Version: 7
  • Priority: P3
  • Status: Closed
  • Resolution: Fixed
  • OS: generic
  • CPU: generic
  • Submitted: 2007-01-08
  • Updated: 2013-11-01
  • Resolved: 2011-03-08
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JDK 6 JDK 7 Other
6u10Fixed 7Fixed hs11Fixed
Related Reports
Relates :  
JDK-6582890 - assert(cost >= 0,"negative spill cost")
Relates :  
JDK-6584342 - assert(hd->head()->is_Loop(),"must begin with loop head node")
Relates :  
JDK-6843752 - missing code for an anti-dependent Phi in GCM
Relates :  
JDK-6586272 - uncommon basic blocks not moved to end
Description
The function PhaseCFG::Estimate_Block_Frequency computes blocks frequencies based on IfNode (and other) probabilities.  Currenntly, the function stops after two passes, and allows inconsistencies in loops of nesting depths of greater than one, among other places.

Both GCM and Block:is_uncommon() rely on the relative frequencies of blocks in order to make cost/benefit choices in their respective optimizations.

This bug calls for consistency in the estimated frequencies, such that the relative frequencies between two blocks be sane, e.g., frequencies inside a loop be greater than that of the loop head.
Comments
EVALUATION Revise Estimate_Block_Frequency to leverage a loop tree to compute estimated frequencies. In its new form, the estimatation function first builds a loop tree. Second, The loop depth is then computed for each loop in the tree. Third, in a bottom up traversal of the tree, compute the frequency of each block in a loop, relative to a single loop entry. Finally, walk the loop tree in a top down fashion, adjusting the frequencies of each loop/block to be relative to a single method entry.
12-07-2007
SUGGESTED FIX See PRT webrev: http://prt-web.sfbay.sun.com/net/prt-archiver.sfbay/data/archived_workspaces/2007/20070712074224.rasbold.c2_baseline0/workspace/webrevs/webrev-2007.07.12/index.html
12-07-2007