JDK-8001336 : C2: Implement Bitwise Liveness Analysis
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: compiler
  • Affected Version: hs25,9,10
  • Priority: P4
  • Status: Open
  • Resolution: Unresolved
  • Submitted: 2012-10-23
  • Updated: 2021-04-27
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Related Reports
Relates :  
JDK-8001436 - C2: Implement bitwise constant propagation
Description
In the long term, we should think about adding bitwise liveness analysis into the system, instead of using pattern matching.  If the live bits out of a LoadB are contained in 0xFF, then we can transform to LoadUB.

Doing this would require a type system that can represent per-bit liveness, and an analysis phase that pushes such types backwards through the graph.  That is a straightforward extension of our existing type system, augmenting the existing arithmetic range endpoints with corresponding bit masks.

��� John
Comments
Here is an example, adapted from a Panama benchmark posted by Maurizio: public int align_loop() { int sum = 0; int i1 = 0; // T_i1 = Int(0,0,0,0) //lo/hi/dn/up int i2 = phi(i1, __); for (;;) { // T_i2 = Int(0, (ES-1)&~AM, 0, BITSUPP(ES-1)&~AM) int i3 = i2 + AL; // T_i3 = Int(0, T_i2.hi + AL, 0, BITSUPP(T_i2.up + AL)&~AM) if (!(i3 < ES)) break; // T_i3 = T_i2 int tval = i3 & AM; // T_tval = Int(0,0,0,0) if (tval != 0) // if (0 != 0) { throw new AssertionError(); } sum += i; i2 = phi(i1, i3); } return sum; } // where Int(lo,hi,dn,up) := // type of x:int requiring // lo<=x && x<=hi && (~x|dn)==-1 && (~x&up)==0 // where BITSUPP(x) := HOBIT(x)*2 - 1 -- bit support mask // where HOBIT(x) := 1<<floor(lg(x)) -- highest one bit // Internal formula used to derive T_i3 in CCP: // T(x+y) := Int(lo, hi, dn, up) where // T_x = T(x), T_y = T(y), // lo = T_x.lo + T_y.lo, up = T_x.lo + T_y.up, --(ignoring overflow!) // p = T_x.up + T_y.up, q = T_x.dn ^ T_x.up, r = T_y.dn ^ T_y.up, // dn = p&~(q|r), up = p|q|r
28-05-2020
Back propagation of bitwise types may provide a framework for liveness analysis. Non-live bits can be represented as top (_dn = 1, _up = 0).
24-10-2012
Related changes: http://hg.openjdk.java.net/hsx/hotspot-comp/hotspot/rev/67f4c477c9ab
23-10-2012