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- Many stylistic nits.
- Removed blkmerge().
- Some minor bug fixes.
- GCM reassoc is now "sink"; a pass that
moves trivial ops in their target block
with the same goal of reducing register
pressure, but starting from instructions
that benefit from having their inputs
close.
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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()
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Comparisons return a 1-bit value, in theory
we could add a Wu1 width for them but I did
not bother and just used Wub. This simply
means that if a frontend generates an extsb
of a comparison result (silly), we will not
generate good code.
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Crashing loads of uninitialized memory
proved to be a problem when implementing
unions using qbe. This patch introduces
a new UNDEF Ref to represent data that is
known to be uninitialized. Optimization
passes can make use of it to eliminate
some code. In the last compilation stages,
UNDEF is treated as the constant 0xdeaddead.
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Previously, we would skip ssa construction when
a temporary has a single definition. This is
only part of the ssa invariant: we must also
check that all uses are dominated by the single
definition. The new code does this.
In fact, qbe does not store all the dominators
for a block, so instead of walking the idom
linked list we use a rough heuristic and declare
conservatively that B0 dominates B1 when one of
the two conditions is true:
a. B0 is the start block
b. B0 is B1
Some measurements on a big file from Michael
Forney show that the code is still as fast as
before this patch.
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If a temporary is assigned exactly
once (most are), there is no need
to do any work to put it in ssa
form.
On an input file of ~35k loc, this
makes the processing time go from
2.9 secs to 1.2 secs.
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Compiler warned about comparison between signed and unsigned values.
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Before this commit, I tried to make sure that
two interfering temporaries never ended up in
the same phi class.
This was to make sure that their register hints
were not counterproductively stepping on each
other's toes. The idea is fine, but:
* the implementation is clumsy because it
mixes the orthogonal concepts of
(i) interference and (ii) phi classes;
* the hinting process in the register
allocator is hard to understand because
the disjoint-set data structure used for
phi classes is cut in arbitrary places.
After this commit, the phi classes *really* are
phi classes represented with a proper disjoint-set
data structure.
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