introduce no-return or unreachable heuristics.
The return heuristics from the Ball and Larus paper don't work well in
practice as they pessimize early return paths. The only good hitrate
return heuristics are those for:
- NULL return
- Constant return
- negative integer return
Only the last of these three can possibly require significant code for
the returning block, and even the last is fairly rare and usually also
a constant. As a consequence, even for the cold return paths, there is
little code on that return path, and so little code density to be gained
by sinking it. The places where sinking these blocks is valuable (inner
loops) will already be weighted appropriately as the edge is a loop-exit
branch.
All of this aside, early returns are nearly as common as all three of
these return categories, and should actually be predicted as taken!
Rather than muddy the waters of the static predictions, just remain
silent on returns and let the CFG itself dictate any layout or other
issues.
However, the return heuristic was flagging one very important case:
unreachable. Unfortunately it still gave a 1/4 chance of the
branch-to-unreachable occuring. It also didn't do a rigorous job of
finding those blocks which post-dominate an unreachable block.
This patch builds a more powerful analysis that should flag all branches
to blocks known to then reach unreachable. It also has better worst-case
runtime complexity by not looping through successors for each block. The
previous code would perform an N^2 walk in the event of a single entry
block branching to N successors with a switch where each successor falls
through to the next and they finally fall through to a return.
Test case added for noreturn heuristics. Also doxygen comments improved
along the way.
llvm-svn: 142793
instructions.
This doesn't introduce any optimizations we weren't doing before (except
potentially due to pass ordering issues), now passes will eliminate them sooner
as part of their own cleanups.
llvm-svn: 142787
a single class. Previously it was split between two classes, one
internal and one external. The concern seemed to center around exposing
the weights used, but those can remain confined to the implementation
file.
Having a single class to maintain the state and analyses in use will
also simplify several of the enhancements I want to make to our static
heuristics.
llvm-svn: 142783
to bring it under direct test instead of merely indirectly testing it in
the BlockFrequencyInfo pass.
The next step is to start adding tests for the various heuristics
employed, and to start fixing those heuristics once they're under test.
llvm-svn: 142778
discussions with Andy. Fundamentally, the previous algorithm is both
counter productive on several fronts and prioritizing things which
aren't necessarily the most important: static branch prediction.
The new algorithm uses the existing loop CFG structure information to
walk through the CFG itself to layout blocks. It coalesces adjacent
blocks within the loop where the CFG allows based on the most likely
path taken. Finally, it topologically orders the block chains that have
been formed. This allows it to choose a (mostly) topologically valid
ordering which still priorizes fallthrough within the structural
constraints.
As a final twist in the algorithm, it does violate the CFG when it
discovers a "hot" edge, that is an edge that is more than 4x hotter than
the competing edges in the CFG. These are forcibly merged into
a fallthrough chain.
Future transformations that need te be added are rotation of loop exit
conditions to be fallthrough, and better isolation of cold block chains.
I'm also planning on adding statistics to model how well the algorithm
does at laying out blocks based on the probabilities it receives.
The old tests mostly still pass, and I have some new tests to add, but
the nested loops are still behaving very strangely. This almost seems
like working-as-intended as it rotated the exit branch to be
fallthrough, but I'm not convinced this is actually the best layout. It
is well supported by the probabilities for loops we currently get, but
those are pretty broken for nested loops, so this may change later.
llvm-svn: 142743
element types, even though the element extraction code does. It is surprising
that this bug has been here for so long. Fixes <rdar://problem/10318778>.
llvm-svn: 142740
able to constant fold load instructions where the argument is a constant.
Second, we should be able to watch multiple PHI nodes through the loop; this
patch only supports PHIs in loop headers, more can be done here.
With this patch, we now constant evaluate:
static const int arr[] = {1, 2, 3, 4, 5};
int test() {
int sum = 0;
for (int i = 0; i < 5; ++i) sum += arr[i];
return sum;
}
llvm-svn: 142731
that the set of callee-saved registers is correct for the specific platform.
<rdar://problem/10313708> & ctor_dtor_count & ctor_dtor_count-2
llvm-svn: 142706
The assumption in the back-end is that PHIs are not allowed at the start of the
landing pad block for SjLj exceptions.
<rdar://problem/10313708>
llvm-svn: 142689
Next step in the ongoing saga of NEON load/store assmebly parsing. Handle
VLD1 instructions that take a two-register register list.
Adjust the instruction definitions to only have the single encoded register
as an operand. The super-register from the pseudo is kept as an implicit def,
so passes which come after pseudo-expansion still know that the instruction
defines the other subregs.
llvm-svn: 142670
ZExtPromotedInteger and SExtPromotedInteger based on the operation we legalize.
SetCC return type needs to be legalized via PromoteTargetBoolean.
llvm-svn: 142660
it's a bit more plausible to use this instead of CodePlacementOpt. The
code for this was shamelessly stolen from CodePlacementOpt, and then
trimmed down a bit. There doesn't seem to be much utility in returning
true/false from this pass as we may or may not have rewritten all of the
blocks. Also, the statistic of counting how many loops were aligned
doesn't seem terribly important so I removed it. If folks would like it
to be included, I'm happy to add it back.
This was probably the most egregious of the missing features, and now
I'm going to start gathering some performance numbers and looking at
specific loop structures that have different layout between the two.
Test is updated to include both basic loop alignment and nested loop
alignment.
llvm-svn: 142645
block frequency analyses. This differs substantially from the existing
block-placement pass in LLVM:
1) It operates on the Machine-IR in the CodeGen layer. This exposes much
more (and more precise) information and opportunities. Also, the
results are more stable due to fewer transforms ocurring after the
pass runs.
2) It uses the generalized probability and frequency analyses. These can
model static heuristics, code annotation derived heuristics as well
as eventual profile loading. By basing the optimization on the
analysis interface it can work from any (or a combination) of these
inputs.
3) It uses a more aggressive algorithm, both building chains from tho
bottom up to maximize benefit, and using an SCC-based walk to layout
chains of blocks in a profitable ordering without O(N^2) iterations
which the old pass involves.
The pass is currently gated behind a flag, and not enabled by default
because it still needs to grow some important features. Most notably, it
needs to support loop aligning and careful layout of loop structures
much as done by hand currently in CodePlacementOpt. Once it supports
these, and has sufficient testing and quality tuning, it should replace
both of these passes.
Thanks to Nick Lewycky and Richard Smith for help authoring & debugging
this, and to Jakob, Andy, Eric, Jim, and probably a few others I'm
forgetting for reviewing and answering all my questions. Writing
a backend pass is *sooo* much better now than it used to be. =D
llvm-svn: 142641
AsmParser. This patch adds validation for target data layout strings upon
construction of TargetData objects. An attempt to construct a TargetData object
from a malformed string will trigger an assertion.
llvm-svn: 142605
When checking the availability of instructions using the TLI, a 'promoted'
instruction IS available. It means that the value is bitcasted to another type
for which there is an operation. The correct check for the availablity of an
instruction is to check if it should be expanded.
llvm-svn: 142542
On spec/gcc, this caused a codesize improvement of ~1.9% for ARM mode and ~4.9% for Thumb(2) mode. This is
codesize including literal pools.
The pools themselves doubled in size for ARM mode and quintupled for Thumb mode, leaving suggestion that there
is still perhaps redundancy in LLVM's use of constant pools that could be decreased by sharing entries.
Fixes PR11087.
llvm-svn: 142530
Add a paste operator '#' to take two identifier-like strings and joint
them. Internally paste gets represented as a !strconcat() with any
necessary casts to string added.
This will be used to implement basic for loop functionality as in:
for i = [0, 1, 2, 3, 4, 5, 6, 7] {
def R#i : Register<...>
}
llvm-svn: 142525
Stop parsing a value if we are in name parsing mode and we see a left
brace. A left brace indicates the start of an object body when we are
parsing a name.
llvm-svn: 142521
Add a mode control to value and ID parsers. The two modes are:
- Parse a value. Expect the parsed ID to map to an existing object.
- Parse a name. Expect the parsed ID to not map to any existing object.
The first is used when parsing an identifier to be looked up, for
example a record field or template argument. The second is used for
parsing declarations. Paste functionality implies that declarations
can contain arbitrary expressions so we need to be able to call into
the general value parser to parse declarations with paste operators.
So we need a way to parse a value-like thing without expecting that
the result will map to some existing object. This parse mode provides
that.
llvm-svn: 142519
Add a Value named "NAME" to each Record. This will be set to the def or defm
name when instantiating multiclasses. This will replace the #NAME# processing
hack once paste functionality is in place.
llvm-svn: 142518
Use lookahead to determine whether a number is really a number or is
part of something forming an identifier. This won't come into play
until the paste operator is recognized as a unique token.
llvm-svn: 142513
Add a peek function to let the Lexer look at a character arbitrarily
far ahead in the stream without consuming anything. We need this to
disambiguate numbers and operands of a paste operation. For example:
def foo#8i
Without lookahead the lexer will treat '8' as a number rather than as
part of a string to be pasted to form an identifier.
llvm-svn: 142512
Add Record names to be changed even on Records that aren't yet
registered. We need to be able to do this for paste functionality
because we do not want to register def names before they are unique
and that can only happen once all paste operations are done. This
change lets us update Record names formed by paste operations and
register the result later.
llvm-svn: 142510
Allow template arg names to be Inits. This is further work to
implement paste as it allows template names to participate in paste
operations.
llvm-svn: 142500
Convert SetValue to take the value name as an Init. This allows us to
set values for variables whose names are not yet fully resolved.
llvm-svn: 142499
Add a couple of utility functions to take a variable name and qualify
it with the namespace of the enclosing class and/or multiclass. This
is inpreparation for making template arg names first-class Inits.
llvm-svn: 142498
Make the VarInit name an Init itself. We need this to implement paste
functionality so we can reference variables whose names are not yet
completely resolved.
llvm-svn: 142497
Add accessors to get Record values by Init name. This lets us look up
Record values whose names are not yet fully resolved. More work
toward paste.
llvm-svn: 142496
and switches, with arbitrary numbers of successors. Still optimized for
the common case of 2 successors for a conditional branch.
Add a test case for switch metadata showing up in the BlockFrequencyInfo pass.
llvm-svn: 142493
encoding of probabilities. In the absense of metadata, it continues to
fall back on static heuristics.
This allows __builtin_expect, after lowering through llvm.expect
a branch instruction's metadata, to actually enter the branch
probability model. This is one component of resolving PR2577.
llvm-svn: 142492
layer already had support for printing the results of this analysis, but
the wiring was missing.
Now that printing the analysis works, actually bring some of this
analysis, and the BranchProbabilityInfo analysis that it wraps, under
test! I'm planning on fixing some bugs and doing other work here, so
having a nice place to add regression tests and a way to observe the
results is really useful.
llvm-svn: 142491
register and then compare against that" method when it's too large. We have to
move the value into the register in the "movw, movt" pair of instructions.
llvm-svn: 142440
register and then compare against that" method when it's too large. We have to
move the value into the register in the "movw, movt" pair of instructions.
llvm-svn: 142437
Clean up the patterns, fix comments, and avoid confusing both tools
and coders. Note that the special adds/subs SelectionDAG nodes no
longer have the dummy cc_out operand.
llvm-svn: 142397
predecessor to remove the jump to it as well. Delay clearing the 'landing pad'
flag until after the jumps have been removed. (There is an implicit assumption
in several modules that an MBB which jumps to a landing pad has only two
successors.)
<rdar://problem/10304224>
llvm-svn: 142390
-Fix binary codes and rename operands in .td files so that automatically
generated function MipsCodeEmitter::getBinaryCodeForInstr gives correct
encoding for instructions.
-Define new class FMem for instructions that access memory.
-Define new class FFRGPR for instructions that move data between GPR and
FPU general and control registers.
-Define custom encoder methods for memory operands, and also for size
operands of ext and ins instructions.
-Only static relocation model is currently implemented.
Patch by Sasa Stankovic
llvm-svn: 142378
Eli Friedman