[M3devel] FW: optimizing range checks?

[Tony Hosking]

The front-end is supposed to be machine-independent.  It has no idea what the underlying machine will do on overflow.

On 2 Mar 2010, at 08:16, Jay K wrote:

> Hey that's pretty clever.
> It costs a register, but given:
>  
>     if (b >= constantX|| b <= -constantY)
>         a = 0;
>  
> The C compiler instead does something like:
>   if ((b + constantY - 1) > (constantX + constantY - 1))
>     a = 0;
>  
> This is something the front end could do in many cases.?
> Adding a constant to eliminate one of the compares and branches is a win.
> If an x86 compiler will give up a register for this, then it is probably a win everywhere.
>  
> Granted, it probably requires silent overflow. Oh well.
>  
>  - Jay
>  
> > Date: Tue, 2 Mar 2010 13:59:31 +0000
> > To: m3commit at elegosoft.com
> > From: jkrell at elego.de
> > Subject: [M3commit] CVS Update: cm3
> > 
> > CVSROOT: /usr/cvs
> > Changes by: jkrell at birch. 10/03/02 13:59:31
> > 
> > Modified files:
> > cm3/m3-libs/m3core/src/Csupport/Common/: hand.c 
> > 
> > Log message:
> > remove m3_shift64, which for the record was:
> > uint64 __stdcall m3_shift64(uint64 a, int b)
> > {
> > if (b >= 64 || b <= -64)
> > a = 0;
> > else if (b > 0)
> > a <<= b;
> > else if (b < 0)
> > a >>= -b;
> > return a;
> > }
> > 
> > _m3_shift64 at 12:
> > 00000000: 55 push ebp
> > 00000001: 8B EC mov ebp,esp
> > 00000003: 8B 4D 10 mov ecx,dword ptr [ebp+10h]
> > 00000006: 8D 41 3F lea eax,[ecx+3Fh]
> > 00000009: 83 F8 7E cmp eax,7Eh
> > 0000000C: 77 1C ja 0000002A
> > 0000000E: 8B 45 08 mov eax,dword ptr [ebp+8]
> > 00000011: 8B 55 0C mov edx,dword ptr [ebp+0Ch]
> > 00000014: 85 C9 test ecx,ecx
> > 00000016: 7E 07 jle 0000001F
> > 00000018: E8 00 00 00 00 call __allshl
> > 0000001D: EB 0F jmp 0000002E
> > 0000001F: 7D 0D jge 0000002E
> > 00000021: F7 D9 neg ecx
> > 00000023: E8 00 00 00 00 call __aullshr
> > 00000028: EB 04 jmp 0000002E
> > 0000002A: 33 C0 xor eax,eax
> > 0000002C: 33 D2 xor edx,edx
> > 0000002E: 5D pop ebp
> > 0000002F: C2 0C 00 ret 0Ch
> > 

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