RFR: 8359424: Eliminate table lookup in Integer/Long toHexString

[Francesco Nigro]

On Tue, 7 Jan 2025 10:39:18 GMT, Shaojin Wen <swen at openjdk.org> wrote:

> In PR #22928, UUID introduced long-based vectorized hexadecimal to string conversion, which can also be used in Integer::toHexString and Long::toHexString to eliminate table lookups. The benefit of eliminating table lookups is that the performance is better when cache misses occur.

I think that without proper assembly analysis won't be easy to check why...


And yes, pdep is bad in old Ryzen @SirYwell :"( 

It could be either a branch prediction problem too (perfnorm would help) if the list of longs can produce small/big hex strings

src/java.base/share/classes/jdk/internal/util/HexDigits.java line 199:

> 197: 
> 198:     /**
> 199:      * Extract the least significant 8 bytes from the input integer i, convert each byte into its corresponding 2-digit

The least significant 4 bytes

src/java.base/share/classes/jdk/internal/util/HexDigits.java line 204:

> 202:      */
> 203:     public static long hex8(long i) {
> 204:         long x = Long.expand(i, 0x0F0F_0F0F_0F0F_0F0FL);

x86 should use  pepd - but aarch64?

src/java.base/share/classes/jdk/internal/util/HexDigits.java line 228:

> 226:         return ((m << 1) + (m >> 1) - (m >> 4))
> 227:                 + 0x3030_3030_3030_3030L
> 228:                 + (x & 0x0F0F_0F0F_0F0F_0F0FL);

x is already expanded at 0x0F0F_0F0F_0F0F_0F0FL, why & it again?

Another thing: IDK how C2 does math here, but on the assembly it should be straightforward to check if we have some register data dep while performing these series of addition/subtraction.
Usually x86 is more affected by this since it has less register available

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PR Comment: https://git.openjdk.org/jdk/pull/22942#issuecomment-2577178403
PR Review Comment: https://git.openjdk.org/jdk/pull/22942#discussion_r1906110618
PR Review Comment: https://git.openjdk.org/jdk/pull/22942#discussion_r1906104894
PR Review Comment: https://git.openjdk.org/jdk/pull/22942#discussion_r1906103700