[llvm-dev] [RFC] Value Range Based Optimization Opportunity in LLVM
Tony Jiang via llvm-dev
llvm-dev at lists.llvm.org
Wed Sep 6 08:43:01 PDT 2017
Hi Daniel,
Thank you very much for you valuable feedback. We are looking into the
range analysis you mentioned. Once we have a better understand about it,
we can discuss more in the follow-up emails.
Regards,
Tony Jiang, M.Sc.
LLVM PPC Backend Development
IBM Toronto Lab, C2/712/8200/MKM
8200 Warden Ave, Markham, L6G 1C7
Email: jtony at ca.ibm.com
Phone: 905-413-3676
From: Daniel Berlin <dberlin at dberlin.org>
To: Hal Finkel <hfinkel at anl.gov>
Cc: Tony Jiang <jtony at ca.ibm.com>, llvm-dev <llvm-dev at lists.llvm.org>
Date: 08/31/2017 10:08 PM
Subject: Re: [llvm-dev] [RFC] Value Range Based Optimization
Opportunity in LLVM
NewGVN does some of it.
It will discover if backpropagation through a phi enables an operation to
be put in terms of existing expressions or constants.
It does not track ranges though, only explicit values.
In your ORIG block, if LENGTH can be expressed as a merge of
already-computed expressions or constants in the program, it will
discover it.
You can stare at examples of this (and compute quite complicated ones if
you wish) at test/Transforms/NewGVN/completeness.ll
Both your examples already have a phi in it in llvm ir, so that is just a
matter of the proper range propagation.
It is really:
int test()
{
int Ret0 = 0;
if (!Ptr)
Ret1= calli(a)
RetPhi = PHI(Ret0, Ret1)
return RetPhi;
}
It should already be possible to determine the value of Ret0 to be 0 based
on ranges without duplicating anything, and the extra return doesn't give
you anything.
If you want to do it based on ranges: a combination of a value range
computation using the existing sparse propagation engine, and a transform
at phi nodes like we use for NewGVN, should be able to handle what you
want here. The existing lazy value info may not get an optimal answer
for various reasons (it is trying to propagate in the wrong direction, so
it loses some context)
You could also use the same basic infrastructure that predicateinfo uses,
and rename info at points the ranges could have changed to get additional
precision.
(unlike predicateinfo, which renames when there are branches).
There are quite a number of papers on making SSA forms that are effective
at range propagation. PredicateInfo is building pruned e-ssa, which is the
basis of a number of range analysis (see
http://homepages.dcc.ufmg.br/~fernando/publications/papers/CGO13_raphael.pdf
and the slides at
http://laure.gonnord.org/pro/research/ER03_2015/RangeAnalysis.pdf).
The only case you should ever need to duplicate code is when the
expression is something not already computed in the program. That's not
the case for your first example, it can be done without code duplication.
For your second example, it would require insertion, but you could know
ahead of time which computations you actually need to insert.
On Thu, Aug 31, 2017 at 6:22 PM, Hal Finkel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
On 08/31/2017 03:54 PM, Tony Jiang via llvm-dev wrote:
Hi All,
We have recently found some optimization opportunities created by
replicating code into branches in order to enable optimization. In
general, the optimization opportunity we are pursuing is like the
following.
Given pseudo-code:
// block A
if (some condition)
// block B
// block C
If it can be efficiently proven that some portion of block C can be
simplified had control flow not gone through the if statement, it might be
useful to convert this CFG into a diamond and hoist that portion of block
C into both block B and the new block.
Consider the following example:
int test(int *Ptr, int a, int b, int c, int d) {
int Ret = 0;
if (__builtin_expect(!Ptr, 0)) {
Ret = calli(a);
// return Ret / (a|1) / (b|1) / (c|1) / (d|1); // Copy return to here
}
return Ret / (a|1) / (b|1) / (c|1) / (d|1); // This can be simplified to
return 0
}
In this case, replicating the return statement in the branch allows the
optimizer to prove the value of Ret at the end of the function is 0 and
eliminate the arithmetic calculations.
A second example:
unsigned long funcReturningArbitraryi64(unsigned char *p);
#define LENGTH(uv) ( (uv) < 0x80 ? 1 : \
(uv) < 0x800 ? 2 : \
(uv) < 0x10000 ? 3 : \
(uv) < 0x200000 ? 4 : \
(uv) < 0x4000000 ? 5 : \
(uv) < 0x80000000 ? 6 : 7 )
int func(unsigned char *p, bool flag)
{
unsigned long c = *p;
int len;
// ...
#ifdef _ORIG
if(flag) {
// ...
c = funcReturningArbitraryi64(p);
}
len = LENGTH(c);
#else
if(flag) {
// ...
c = funcReturningArbitraryi64(p);
len = LENGTH(c);
} else {
len = LENGTH(c);
}
#endif
// ...
return len;
}
In this case, we see that creating an else block and replicating the
return statement in both the if and else branch (like the code snippet
guarded by the #else) enables the macro UNISKIP in the else branch to be
optimized.
Most of the examples we have come up with so far are centered around value
ranges along the conditional branches. When the range of values a symbol
can have along different branches is provably different, opportunities for
optimization may arise. However, this likely isn't the only category of
optimizations that could benefit from this.
Is there an existing transformation in LLVM that should be doing this
already that is missing this opportunity? If not, we would like to pursue
adding this. Of course, this optimization would be subject to a cost model
as it may result in code growth. For example, it may not be advantageous
to do this if the simplified branch is cold. If anyone has any
comments/suggestions we are very much interested in hearing them.
We have two transformations that track ranges along CFG edges using
LazyValueInfo: JumpThreading and CorrelatedValuePropagation. I don't think
that either does what you're proposing.
-Hal
Regards,
Tony Jiang, M.Sc.
LLVM PPC Backend Development
IBM Toronto Lab, C2/712/8200/MKM
8200 Warden Ave, Markham, L6G 1C7
Email: jtony at ca.ibm.com
Phone: 905-413-3676
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--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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