Re: [Tarantool-patches] [PATCH luajit 4/5] Fix pow() optimization inconsistencies.

[Sergey Kaplun via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

Hi, Maxim!
Thanks for the review!
See my answers below.

On 20.08.23, Maxim Kokryashkin wrote:
> Hi, Sergey!
> Thanks for the patch!
> Please consider my comments below.
> 
> On Tue, Aug 15, 2023 at 12:36:30PM +0300, Sergey Kaplun wrote:
> > From: Mike Pall <mike>
> > 
> > (cherry-picked from commit 9512d5c1aced61e13e7be2d3208ec7ae3516b458)
> > 
> > This patch fixes different misbehaviour between JIT-compiled code and
> Typo: s/misbehaviour/misbehaviours/

Fixed.

> > the interpreter for power operator with the following ways:
> Typo: s/with the/in the/

Fixed.

> > * Drop folding optimizations for base ^ 0.5 => sqrt(base), as far as
> >   pow(base, 0.5) isn't interchangeable and depends on the <math.h>
> >   implementation.
> > * Drop folding optimizations for 2 ^ int_pow => ldexp(1.0, int_pow), to
> >   avoid dependcy on the <math.h> implementation.
> > * Now `asm_pow()` always assemble a call to the `lj_vm_powi()` function,
> Typo: s/assemble/assembles/

Fixed.

> >   that is general now for all CPU architectures. Using this internal
> >   function instead of toolchain-provided `pow()` guarantees consistency
> Typo: s/of/of the/

Fixed.

> >   between interpreter and JIT results. Also, it drops custom
> Typo: s/drops/drops the/

Fixed.

> >   implementation for the `vm_powi_sse()` on x86_64.
> Typo: s/for the/for/

Fixed.

> > * `math_extern2` macro in the VM may take the second argument, that is
> >   used as the target function to call. The first argument is still the
> >   name for `func_nnsse` macro.
> > * Narrowing for power operation avoids range guard for non-constant base
> >   IR. This leads to invalid result if value on trace is out of range.
> Typo: s/to invalid/to an invalid/

Fixed.

> >   Now it is done unconditionally.
> > 
> > Be aware, that [220/502] lib/string/format/num.lua test [1] from
> Typo: s/from the/from/

I suppose that it should be "from the"? Fixed.

> > LuaJIT-test suite fails after this commit.
> > 
> > [1]: https://www.exploringbinary.com/incorrect-floating-point-to-decimal-conversions/
> > 
> > Sergey Kaplun:
> > * added the description and the test for the problem
> > 
> > Part of tarantool/tarantool#8825
> > ---

<snipped>

> > +local res = {}
> > +-- -0 ^ 0.5 = 0. Test sign with `tostring()`.
> Typo: s/Test/Test the/

Fixed.

> > +-- XXX: use local variable to prevent folding via parser.

<snipped>

> > +
> > +-- 2921 ^ 0.5 = 0x1.b05ec632536fap+5.
> We certainly need to add some explanation here about the precision, because
> it is not obvious why these magic numbers should cause any issues.

I suppose any really intererested in this reader may compare the
behaviour of the glibc implementation of `sqrt()` and `pow()`. Also, the
comment should mention this implementation, so it becomes too huge and
distracts the reader from the test case itself.

Ignoring for now.

> > +res = {}

<snipped>

> > +test:samevalues(res, ('consistent results for folding 2921 ^ 0.5'))
> 
> I believe it is possible to make a single function with different
> parameters for all three cases above.
> Something like `test_power(value, power, extra_map)`, so you can do
> | res[i] = extra_map(value ^ power)

I afraid that this function doesn't give any improvement in readability,
also, it may change the trace semantics, so I prefer to leave it as is.

Ignoring for now.

> 
> > +

<snipped>

> > +-- Need some value near 1, to avoid infinite result.
> Typo: s/Need/We need/
> Typo: s/avoid/avoid an/

Fixed.

See the iterative patch below.

===================================================================
diff --git a/test/tarantool-tests/lj-684-pow-inconsistencies.test.lua b/test/tarantool-tests/lj-684-pow-inconsistencies.test.lua
index 5129fc45..003fe957 100644
--- a/test/tarantool-tests/lj-684-pow-inconsistencies.test.lua
+++ b/test/tarantool-tests/lj-684-pow-inconsistencies.test.lua
@@ -18,7 +18,7 @@ jit.off()
 jit.flush()
 
 local res = {}
--- -0 ^ 0.5 = 0. Test sign with `tostring()`.
+-- -0 ^ 0.5 = 0. Test the sign with `tostring()`.
 -- XXX: use local variable to prevent folding via parser.
 -- XXX: use stack slot out of trace to prevent constant folding.
 local minus_zero = -0
@@ -75,7 +75,7 @@ jit.on()
 pow(1, 2)
 pow(1, 2)
 
--- Need some value near 1, to avoid infinite result.
+-- We need some value near 1, to avoid an infinite result.
 local base = 1.0000000001
 local power = 65536 * 3
 local resulting_value = pow(base, power)
===================================================================

> > +local base = 1.0000000001

<snipped>

> > -- 
> > 2.41.0
> > 

-- 
Best regards,
Sergey Kaplun