Why doesn't HotSpot use div machine code?

[Vladimir Kozlov]

x86 instructions latency:

idiv - 56-70
imul - 14

14(imul)+14(imul)+5(0.5 per simple instruction) much less then lowest 56.

Vladimir

Ulf Zibis wrote:
> 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
> 
>