Why doesn't HotSpot use div machine code?

[Osvaldo Doederlein]

Hi,

Perhaps because of this:
http://support.amd.com/us/Processor_TechDocs/40546-PUB-Optguide_3-11_5-21-09.pdf

imul's latencies are tiny (3 cycles for both forms used in the code), but
div/idiv's are enormous (check Table 7). These numbers are for a specific
CPU family but I don't expect this to be very different in other CPUs. The
code produce by HotSpot will probably win, even with the extra shifts, movs
etc.

OTOH, I wonder if HotSpot would be capable to produce your desired code if
it was faster - it consumes less registers, and that's also very important
remarkably in x86.

A+
Osvaldo

2009/12/23 Ulf Zibis <Ulf.Zibis at gmx.de>

> In my code I have a method similar to the following:
> (divide char value by 8-bit constant and combine it's lower 8-bit quotient
> and remainder to a new char value)
>
>    static final byte BYTE_RANGE = 0x5e;
>    static char db(char db) {
>       return (char)((((db / (BYTE_RANGE&0xff) & 0xff) << 8) | (db %
> (BYTE_RANGE&0xff) & 0xff)) // force DIV word/byte
>               + ...;
>   }
>
> This could be compiled to:
>
> mov    %cx,%ax    ; copy char db to ax register
> div    $0x5e
> xchg   %al,%ah
>
> ... but disassembly output results:
> (some sophisticated trick using 2 imul instructions)
>
>  0x00ba4f67: mov    $0xae4c415d,%eax
>  0x00ba4f6c: imul   %ecx
>  0x00ba4f6e: add    %ecx,%edx          ;*idiv
>                                       ; -
> sun.nio.cs.ext.EUC_TW_C_d_b_c1_f3_shortMap4$Encoder::db at 3 (line 515)
>  0x00ba4f70: mov    %edx,%ebp
>  0x00ba4f72: sar    $0x6,%ebp
>  0x00ba4f75: shr    $0x6,%edx
>  0x00ba4f78: imul   $0x5e,%ebp,%ebp
>  0x00ba4f7b: sub    %ebp,%ecx
>  0x00ba4f7d: and    $0xff,%edx
>  0x00ba4f83: and    $0xff,%ecx
>  0x00ba4f89: shl    $0x8,%edx
>  0x00ba4f8c: or     %ecx,%edx
>  ...
>
> Complete output here (line 2330):
>
> https://java-nio-charset-enhanced.dev.java.net/source/browse/java-nio-charset-enhanced/branches/j7_EUC_TW/log/C_d_b_c1_f3_shortMap4_PA_2.xml?rev=888&view=markup
>
> Why doesn't HotSpot use div machine code?
> I guess this would be faster here.
>
> -Ulf
>
>
>
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