Global Value Numbering / Global Code Motion

More or less as proposed in its ninth iteration with the
addition of a gcmmove() functionality to restore coherent
local schedules.

Changes since RFC 8:

Features:

 - generalization of phi 1/0 detection
 - collapse linear jmp chains before GVN; simplifies if-graph
     detection used in 0/non-0 value inference and if-elim...
 - infer 0/non-0 values from dominating blk jnz; eliminates
     redundant cmp eq/ne 0 and associated jnz/blocks, for example
     redundant null pointer checks (hare codebase likes this)
 - remove (emergent) empty if-then-else graphlets between GVN and
     GCM; improves GCM instruction placement, particularly cmps.
 - merge %addr =l add %addr1, N sequences - reduces tmp count,
     register pressure.
 - squash consecutive associative ops with constant args, e.g.
     t1 = add t, N ... t2 = add t2, M -> t2 = add t, N+M

Bug Fixes:

 - remove "cmp eq/ne of non-identical RCon's " in copyref().
   RCon's are not guaranteed to be dedup'ed, and symbols can
   alias.

Codebase:

  - moved some stuff into cfg.c including blkmerge()
  - some refactoring in gvn.c
  - simplification of reassoc.c - always reassoc all cmp ops
      and Kl add %t, N. Better on coremark, smaller codebase.
  - minor simplification of movins() - use vins

Testing - standard QBE, cproc, hare, harec, coremark
          [still have Rust build issues with latest roland]

Benchmark
- coremark is ~15%+ faster than master
- hare "HARETEST_INCLUDE='slow' make check" ~8% faster
    (crypto::sha1::sha1_1gb is biggest obvious win - ~25% faster)

Changes since RFC 7:

Bug fixes:

- remove isbad4gcm() in GVN/GCM - it is unsound due to different state
    at GVN vs GCM time; replace with "reassociation" pass after GCM
- fix intra-blk use-before-def after GCM
- prevent GVN from deduping trapping instructions cos GCM will not
    move them
- remove cmp eq/ne identical arg copy detection for floating point, it
    is not valid for NaN
- fix cges/cged flagged as commutative in ops.h instead of cnes/cned
    respectively; just a typo

Minor features:

- copy detection handles cmp le/lt/ge/gt with identical args
- treat (integer) div/rem by non-zero constant as non-trapping
- eliminate add N/sub N pairs in copy detection
- maintain accurate tmp use in GVN; not strictly necessary but enables
    interim global state sanity checking
- "reassociation" of trivial constant offset load/store addresses, and
    cmp ops with point-of-use in pass after GCM
- normalise commutative op arg order - e.g. op con, tmp -> op tmp, con
    to simplify copy detection and GVN instruction dedup

Codebase:

- split out core copy detection and constant folding (back) out into
    copy.c, fold.c respectively; gvn.c was getting monolithic
- generic support for instruction moving in ins.c - used by GCM and
    reassoc
- new reassociation pass in reassoc.c
- other minor clean-up/refactor

Changes since RFC 6:

- More ext elimination in GVN by examination of def and use bit width
- elimination of redundant and mask by bit width examination
- Incorporation of Song's patch

Changes since RFC 5:

- avoidance of "bad" candidates for GVN/GCM - trivial address offset
    calculations, and comparisons
- more copy detection mostly around boolean values
- allow elimination of unused load, alloc, trapping instructions
- detection of trivial boolean v ? 1 : 0 phi patterns
- bug fix for (removal of) "chg" optimisation in ins recreation - it
    was missing removal of unused instructions in some cases

ifelim() between GVN and GCM; deeper nopunused()

1 files changed, 43 insertions, 0 deletions

diff --git a/test/_gcm2.ssa b/test/_gcm2.ssa
new file mode 100644
index 0000000..baeb4a4
--- /dev/null
+++ b/test/_gcm2.ssa
@@ -0,0 +1,43 @@
+# Programs from "Global Code Motion Global Value Numbering" by Cliff Click
+# https://courses.cs.washington.edu/courses/cse501/06wi/reading/click-pldi95.pdf
+
+# GCM program in Figure 1
+
+function w $gcm_test(w %a){
+@start
+	%i.0 =w copy 0
+@loop
+	%i.1 =w phi @start %i.0, @loop %i.2
+	%b =w add %a, 1                     # early schedule moves to @start
+	%i.2 =w add %i.1, %b
+	%c =w mul %i.2, 2                   #  late schedule moves to @end
+	%x =w csltw %i.2, 10
+	jnz %x, @loop, @end
+@end
+	ret %c
+}
+
+# GCM program in "Figure 3 x's definition does not dominate it's use"
+#
+# SSA contruction will insert phi instruction for "x" in @if_false
+# preventing the "add" in @if_false from being moved to @if_true
+
+function $gcm_test2 (w %a){
+@start
+	%f =w copy 1
+	%x =w copy 0
+	%s.0 =w copy 0
+@loop
+	%s.1 = w phi @start %s.0, @if_false %s.2
+	jnz %a, @if, @end
+@if
+	jnz %f, @if_true, @if_false
+@if_true
+	%f =w copy 0
+	%x =w add %x, 1
+@if_false
+	%s.2 =w add %s.1, %x
+	jmp @loop
+@end
+	ret
+}