Re: [Tarantool-patches] [PATCH luajit] x86/x64: Fix math.ceil(-0.9) result sign.

[Maxim Kokryashkin via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

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Hi!
Thanks for the fixes!
LGTM now.
 
 
--
Best regards,
Maxim Kokryashkin
 
  
>Вторник, 7 ноября 2023, 13:26 +03:00 от Sergey Kaplun <skaplun@tarantool.org>:
> 
>Hi, Maxim!
>Thanks for the review!
>Fixed typos and force-pushed the branch.
>
>On 07.11.23, Maxim Kokryashkin wrote:
>> Hi, Sergey!
>> Thanks for the patch!
>> LGTM, except for a few nits below.
>>
>> On Thu, Nov 02, 2023 at 03:36:16PM +0300, Sergey Kaplun wrote:
>> > From: Mike Pall <mike>
>> >
>> > Reported by minoki.
>> >
>> > (cherry-picked from commit 674afcd4e21d0cf64de3219d347557a0aed8ecc7)
>> >
>> > The `ceil` (`floor`) math function implementation calculates (|x| +
>> > 2^52) - 2^52 for its argument to determine the fractional part of x , so
>> Typo: s/x ,/x,/
>
>Fixed, thanks!
>
>> > it will be rounded to the nearest integer and its sign is restored.
>> > After that, if the original value is < (>) than the result, the -1 (1)
>> > is subtracted from it. Take a look at the `ceil()` case. The result of
>> > the operation `-1 - (-1)` is +0 for FP arithmetic, against -0 expected
>> > as a result.
>> >
>> > This patch changes the `- (-1)` operation to `+ 1` and restores sign
>> > after it again.
>> >
>> > NB: Since in DUALNUM mode on x86/x64 all results are tried to be
>> > converted to integers the sign of 0 is neglected.
>> Typo: s/integers/integers,/
>
>Fixed!
>
>> >
>> > Sergey Kaplun:
>> > * added the description and the test for the problem
>> >
>> > Part of tarantool/tarantool#9145
>> > ---
>> >
>> > Branch:  https://github.com/tarantool/luajit/tree/skaplun/lj-859-math-ceil-sign
>> > Tarantool PR:  https://github.com/tarantool/tarantool/pull/9326
>> > Related issues:
>> > *  https://github.com/LuaJIT/LuaJIT/issues/859
>> > *  https://github.com/tarantool/tarantool/issues/9145
>> >
>> > src/vm_x64.dasc | 13 ++++++------
>> > src/vm_x86.dasc | 13 ++++++------
>> > .../lj-859-math-ceil-sign.test.lua | 20 +++++++++++++++++++
>> > 3 files changed, 32 insertions(+), 14 deletions(-)
>> > create mode 100644 test/tarantool-tests/lj-859-math-ceil-sign.test.lua
>> >
>> > diff --git a/src/vm_x64.dasc b/src/vm_x64.dasc
>> > index 399dfcbf..fc4c6259 100644
>> > --- a/src/vm_x64.dasc
>> > +++ b/src/vm_x64.dasc
>> > @@ -400,9 +400,6 @@
>> > |.macro sseconst_1, reg, tmp // Synthesize 1.0.
>> > | sseconst_hi reg, tmp, 3ff00000
>> > |.endmacro
>> > -|.macro sseconst_m1, reg, tmp // Synthesize -1.0.
>> > -| sseconst_hi reg, tmp, bff00000
>> > -|.endmacro
>> > |.macro sseconst_2p52, reg, tmp // Synthesize 2^52.
>> > | sseconst_hi reg, tmp, 43300000
>> > |.endmacro
>> > @@ -2598,15 +2595,17 @@ static void build_subroutines(BuildCtx *ctx)
>> > | addsd xmm1, xmm3 // (|x| + 2^52) - 2^52
>> > | subsd xmm1, xmm3
>> > | orpd xmm1, xmm2 // Merge sign bit back in.
>> > + | sseconst_1 xmm3, RD
>> > | .if mode == 1 // ceil(x)?
>> > - | sseconst_m1 xmm2, RD // Must subtract -1 to preserve -0.
>> > | cmpsd xmm0, xmm1, 6 // x > result?
>> > + | andpd xmm0, xmm3
>> > + | addsd xmm1, xmm0 // If yes, add 1.
>> > + | orpd xmm1, xmm2 // Merge sign bit back in (again).
>> > | .else // floor(x)?
>> > - | sseconst_1 xmm2, RD
>> > | cmpsd xmm0, xmm1, 1 // x < result?
>> > + | andpd xmm0, xmm3
>> > + | subsd xmm1, xmm0 // If yes, subtract 1.
>> > | .endif
>> > - | andpd xmm0, xmm2
>> > - | subsd xmm1, xmm0 // If yes, subtract +-1.
>> > |.endif
>> > | movaps xmm0, xmm1
>> > |1:
>> > diff --git a/src/vm_x86.dasc b/src/vm_x86.dasc
>> > index 9fa9a3f7..05a8267b 100644
>> > --- a/src/vm_x86.dasc
>> > +++ b/src/vm_x86.dasc
>> > @@ -533,9 +533,6 @@
>> > |.macro sseconst_1, reg, tmp // Synthesize 1.0.
>> > | sseconst_hi reg, tmp, 3ff00000
>> > |.endmacro
>> > -|.macro sseconst_m1, reg, tmp // Synthesize -1.0.
>> > -| sseconst_hi reg, tmp, bff00000
>> > -|.endmacro
>> > |.macro sseconst_2p52, reg, tmp // Synthesize 2^52.
>> > | sseconst_hi reg, tmp, 43300000
>> > |.endmacro
>> > @@ -3089,15 +3086,17 @@ static void build_subroutines(BuildCtx *ctx)
>> > | addsd xmm1, xmm3 // (|x| + 2^52) - 2^52
>> > | subsd xmm1, xmm3
>> > | orpd xmm1, xmm2 // Merge sign bit back in.
>> > + | sseconst_1 xmm3, RDa
>> > | .if mode == 1 // ceil(x)?
>> > - | sseconst_m1 xmm2, RDa // Must subtract -1 to preserve -0.
>> > | cmpsd xmm0, xmm1, 6 // x > result?
>> > + | andpd xmm0, xmm3
>> > + | addsd xmm1, xmm0 // If yes, add 1.
>> > + | orpd xmm1, xmm2 // Merge sign bit back in (again).
>> > | .else // floor(x)?
>> > - | sseconst_1 xmm2, RDa
>> > | cmpsd xmm0, xmm1, 1 // x < result?
>> > + | andpd xmm0, xmm3
>> > + | subsd xmm1, xmm0 // If yes, subtract 1.
>> > | .endif
>> > - | andpd xmm0, xmm2
>> > - | subsd xmm1, xmm0 // If yes, subtract +-1.
>> > |.endif
>> > | movaps xmm0, xmm1
>> > |1:
>> > diff --git a/test/tarantool-tests/lj-859-math-ceil-sign.test.lua b/test/tarantool-tests/lj-859-math-ceil-sign.test.lua
>> > new file mode 100644
>> > index 00000000..df0ca329
>> > --- /dev/null
>> > +++ b/test/tarantool-tests/lj-859-math-ceil-sign.test.lua
>> > @@ -0,0 +1,20 @@
>> > +local tap = require('tap')
>> > +
>> > +-- Test file to demonstrate the incorrect LuaJIT's behaviour
>> > +-- for `math.ceil(x)` when argument `x`: -1 < x < -0.5.
>> > +-- See also  https://github.com/LuaJIT/LuaJIT/issues/859 .
>> > +
>> > +local test = tap.test('lj-859-math-ceil-sign')
>> > +
>> > +test:plan(1)
>> > +
>> > +local IS_DUALNUM = tostring(tonumber('-0')) ~= tostring(-0)
>> > +local IS_X86_64 = jit.arch == 'x86' or jit.arch == 'x64'
>> > +
>> > +-- Use `tostirng()` to compare sign of the returned value.
>> Typo: s/sign/the sign/
>
>Fixed.
>
>> > +-- Take any value from the mentioned range. Chosen one is
>> Typo: s/Chosen/The chosen/
>
>Fixed.
>
>> > +-- mentioned in the commit message.
>> > +test:ok((IS_DUALNUM and IS_X86_64) or tostring(math.ceil(-0.9)) == '-0',
>> Nit: maybe we should precalculate the predicate before the `test:ok` call?
>> Or, at least the `IS_DUALNUM and IS_X86_64`, as it is essentially a skipcond.
>
>I preferred the following way to accent in one place when the test is
>skipped (in the test itself).
>
>Ignoring for now.
>
>> > + 'correct zero sign')
>> > +
>> > +test:done(true)
>> > --
>> > 2.42.0
>> >
>
>See the iterative patch below, branch is force-pushed.
>
>===================================================================
>diff --git a/test/tarantool-tests/lj-859-math-ceil-sign.test.lua b/test/tarantool-tests/lj-859-math-ceil-sign.test.lua
>index df0ca329..831b6480 100644
>--- a/test/tarantool-tests/lj-859-math-ceil-sign.test.lua
>+++ b/test/tarantool-tests/lj-859-math-ceil-sign.test.lua
>@@ -11,8 +11,8 @@ test:plan(1)
> local IS_DUALNUM = tostring(tonumber('-0')) ~= tostring(-0)
> local IS_X86_64 = jit.arch == 'x86' or jit.arch == 'x64'
> 
>--- Use `tostirng()` to compare sign of the returned value.
>--- Take any value from the mentioned range. Chosen one is
>+-- Use `tostirng()` to compare the sign of the returned value.
>+-- Take any value from the mentioned range. The chosen one is
> -- mentioned in the commit message.
> test:ok((IS_DUALNUM and IS_X86_64) or tostring(math.ceil(-0.9)) == '-0',
>         'correct zero sign')
>===================================================================
>
>--
>Best regards,
>Sergey Kaplun
 

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