[llvm] r248633 - Use fixed-point representation for BranchProbability.

[Cong Hou via llvm-commits]

Thanks for the fix, Benjamin!


thanks,
Cong

On Sat, Sep 26, 2015 at 4:07 AM, Benjamin Kramer <benny.kra at gmail.com>
wrote:

>
> > On 26.09.2015, at 09:13, İsmail Dönmez via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
> >
> > Hi,
> >
> > On Sat, Sep 26, 2015 at 2:09 AM, Cong Hou via llvm-commits
> > <llvm-commits at lists.llvm.org> wrote:
> >> Author: conghou
> >> Date: Fri Sep 25 18:09:59 2015
> >> New Revision: 248633
> >>
> >> URL: http://llvm.org/viewvc/llvm-project?rev=248633&view=rev
> >> Log:
> >> Use fixed-point representation for BranchProbability.
> >>
> >> BranchProbability now is represented by its numerator and denominator
> in uint32_t type. This patch changes this representation into a fixed point
> that is represented by the numerator in uint32_t type and a constant
> denominator 1<<31. This is quite similar to the representation of BlockMass
> in BlockFrequencyInfoImpl.h. There are several pros and cons of this change:
> >>
> >> Pros:
> >>
> >> 1. It uses only a half space of the current one.
> >> 2. Some operations are much faster like plus, subtraction, comparison,
> and scaling by an integer.
> >>
> >> Cons:
> >>
> >> 1. Constructing a probability using arbitrary numerator and denominator
> needs additional calculations.
> >> 2. It is a little less precise than before as we use a fixed
> denominator. For example, 1 - 1/3 may not be exactly identical to 1 / 3
> (this will lead to many BranchProbability unit test failures). This should
> not matter when we only use it for branch probability. If we use it like a
> rational value for some precise calculations we may need another construct
> like ValueRatio.
> >>
> >> One important reason for this change is that we propose to store branch
> probabilities instead of edge weights in MachineBasicBlock. We also want
> clients to use probability instead of weight when adding successors to a
> MBB. The current BranchProbability has more space which may be a concern.
> >>
> >> Differential revision: http://reviews.llvm.org/D12603
> >>
> >>
> >> Modified:
> > [...]
> >>    llvm/trunk/test/Analysis/BranchProbabilityInfo/basic.ll
> >>    llvm/trunk/test/Analysis/BranchProbabilityInfo/loop.ll
> >
> > These two tests fail with MSVC 2015 :
> >
> >
> C:\cygwin64\home\ismail\src\llvm\test\Analysis\BranchProbabilityInfo\basic.ll:18:10:
> > error: expected string not found in input
> > ; CHECK: edge body -> exit probability is 0x04000000 / 0x80000000 = 3.12%
> >         ^
> > <stdin>:4:2: note: scanning from here
> > edge body -> exit probability is 0x04000000 / 0x80000000 = 3.13%
> > ^
> >
> >
> C:\cygwin64\home\ismail\src\llvm\test\Analysis\BranchProbabilityInfo\loop.ll:27:10:
> > error: expected string not found in input
> > ; CHECK: edge do.body1 -> do.end probability is 0x04000000 / 0x80000000
> = 3.12%
> >         ^
> > <stdin>:6:2: note: scanning from here
> > edge do.body1 -> do.end probability is 0x04000000 / 0x80000000 = 3.13%
> > ^
>
> r248665 works around the different rounding behavior of printf on Windows.
>
> - Ben
>
>
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