Why doesn't HotSpot use div machine code?

Ulf Zibis Ulf.Zibis at gmx.de
Wed Dec 23 13:01:13 PST 2009

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):

Why doesn't HotSpot use div machine code?
I guess this would be faster here.


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