RFR: 8349637: Integer.numberOfLeadingZeros outputs incorrectly in certain cases

[Quan Anh Mai]

On Wed, 12 Feb 2025 05:47:52 GMT, Jasmine Karthikeyan <jkarthikeyan at openjdk.org> wrote:

> Hi all,
> This is a fix for a miscompile in the AVX2 implementation of `CountLeadingZerosV` for int types. Currently, the implementation turns ints into floats, in order to calculating the leading zeros based on the exponent part of the float. Unfortunately, floats can only accurately represent integers up to 2^24. After that, multiple integer values can map onto the same floating point value. The issue manifests when an int is converted to a floating point representation that is higher than it, crossing a bit boundary. As an example, `(float)0x01FFFFFF == (float)0x02000000`, but `lzcnt(0x01FFFFFF) == 7` and `lzcnt(0x02000000) == 6`. The values are incorrectly rounded up.
> 
> This patch fixes the issue by masking the input in the cases where it is larger than 2^24, to set the low bits to 0. Removing these bits prevents the accidental rounding behavior. I've added these cases to`TestNumberOfContinuousZeros`, and removed the set random seed so that it can produce random inputs to test with.
> 
> Reviews would be appreciated!

src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp line 6242:

> 6240:   vpxor(xtmp2, xtmp2, xtmp2, vec_enc);
> 6241:   vpsrld(xtmp2, src, 24, vec_enc);
> 6242:   vpandn(src, xtmp2, src, vec_enc);

Are you sure this will work? I don't see how `x &~ (x >> 24)` can zero out all lower bits. Also, we should not kill `src` here.

I think a better solution is to do: `x > 0xFF ? x &~ 0xFF : x`. I chose this value because we have `0xFF` in `xtmp1` here.

test/hotspot/jtreg/compiler/vectorization/TestNumberOfContinuousZeros.java line 62:

> 60:         inputInt = new int[LEN];
> 61:         outputInt = new int[LEN];
> 62:         rng = new Random();

Please use `Util.getRandomInstance()`

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PR Review Comment: https://git.openjdk.org/jdk/pull/23579#discussion_r1952070033
PR Review Comment: https://git.openjdk.org/jdk/pull/23579#discussion_r1952070879