moving int to byte[] in single step

[Ulf Zibis]

internal review ID 1680023

-Ulf

Am 24.12.2009 01:02, Ulf Zibis schrieb:
> In my code I have following snippet:
>
>        byte[] da = ...;
>        int dp = ...;
>        int qb = ...;
>        da[dp+3] = (byte)qb;
>        da[dp+2] = (byte)(qb >> 8);
>        da[dp+1] = (byte)(qb >> 16);
>        da[dp+0] = (byte)(qb >> 24);
>
> This could be compiled to something like:
> mov    0x24(%esp),%edi
> ..
> mov    %eax,%ebx
> bswap  %ebx     ; only little-endian architectures
> mov    0x20(%esp),%esi
> mov    0x8(%esi),%edx     ; implicit exception: dispatches to 0x00ba49e4
> cmp    %edx,%edi
> jae    0x00ba49a5
> mov    0x24(%esp),%ebp
> add    $0x3,%ebp
> cmp    %edx,%ebp
> jae    0x00ba49c1
> mov    %ebx,0xc(%esi,%edi,4)  ;*bastore
> ...
>
> ... but disassembly output results:
> (copies each of the 4 bytes in single steps)
>
>  0x00ba48d6: mov    0x24(%esp),%edi
>  ...
>  0x00ba492a: mov    %eax,%ebx
>  0x00ba492c: sar    $0x18,%ebx         ;*ishr
>                                        ; - 
> sun.nio.cs.ext.EUC_TW_C_d_b_c1_f3_shortMap4$Encoder::encode at 93 (line 491)
>  0x00ba492f: mov    0x20(%esp),%esi
>  0x00ba4933: mov    0x8(%esi),%edx     ; implicit exception: 
> dispatches to 0x00ba49e4
>  0x00ba4936: cmp    %edx,%edi
>  0x00ba4938: jae    0x00ba49a5
>  0x00ba493a: mov    %bl,0xc(%esi,%edi,1)  ;*bastore
>                                        ; - 
> sun.nio.cs.ext.EUC_TW_C_d_b_c1_f3_shortMap4$Encoder::encode at 95 (line 491)
>  0x00ba493e: mov    0x24(%esp),%ebp
>  0x00ba4942: add    $0x3,%ebp
>  0x00ba4945: mov    %eax,%ecx
>  0x00ba4947: sar    $0x10,%ecx         ;*ishr
>                                        ; - 
> sun.nio.cs.ext.EUC_TW_C_d_b_c1_f3_shortMap4$Encoder::encode at 104 (line 
> 492)
>  0x00ba494a: cmp    %edx,%ebp
>  0x00ba494c: jae    0x00ba49c1
>  0x00ba494e: mov    %cl,0xd(%esi,%edi,1)  ;*bastore
>                                        ; - 
> sun.nio.cs.ext.EUC_TW_C_d_b_c1_f3_shortMap4$Encoder::encode at 106 (line 
> 492)
>  0x00ba4952: mov    %eax,%ebx
>  0x00ba4954: sar    $0x8,%ebx
>  0x00ba4957: mov    %bl,0xe(%esi,%edi,1)  ;*bastore
>                                        ; - 
> sun.nio.cs.ext.EUC_TW_C_d_b_c1_f3_shortMap4$Encoder::encode at 117 (line 
> 493)
>  0x00ba495b: mov    %al,0xf(%esi,%edi,1)  ;*bastore
>                                        ; - 
> sun.nio.cs.ext.EUC_TW_C_d_b_c1_f3_shortMap4$Encoder::encode at 125 (line 
> 494)
> ...
>
> Complete output here (line 2497):
> 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 
>
>
> As you can see, copying the 4 bytes could be done at once.
> I guess this would be faster here.
>
> -Ulf
>
>
>