[fpc-devel] Possible error in generated code for arm?

[Michael Ring]

I have the following code in a unit (I need to provide a memset function 
to be able to link to freertos):

function memset(pxBuffer:pointer; value : byte; count : Tsize):pointer; 
cdecl;
begin
   FillChar(pxBuffer,count,value);
   Result := pxBuffer;
end;


When I look at the assembler code generated by latest Trunk fpc 
something is very odd @08000970, I'd expect to see

LDR           R0, [R11, #-0x30]

but in fact I see:

  08000970   SUB.W          R0, R11, #0x30

which makes no sense to me and later the code crashes inside of the 
FillChar routine.

Compiler Bug? Or me overlooking something? I tried to compile with -O- 
and -O1, with -O2 the generated code is a little different but there the 
SP is loaded to R0 which I do not understand, I'd expect R0 to be set to 
address of pxBuffer. (this is for armv7em, for armv6 also looks odd.


Thank you for your help,

Michael

FREERTOS_$$_MEMSET$POINTER$BYTE$LONGWORD$$POINTER
DEBUGSTART_$FREERTOS
memset
$Thumb
begin
  08000958   MOV            R12, SP
  0800095A   PUSH.W         {R11, LR}
  0800095E   SUB.W          R11, R12, #4
  08000962   SUB            SP, SP, #0x48
  08000964   STR            R0, [R11, #-0x30]
  08000968   STRB           R1, [R11, #-0x34]
  0800096C   STR            R2, [R11, #-0x38]
FillChar(pxBuffer,count,value);
  08000970   SUB.W          R0, R11, #0x30
  08000974   LDRB           R2, [R11, #-0x34]
  08000978   LDR            R1, [R11, #-0x38]
  0800097C   BL             SYSTEM_$$_FILLCHAR$formal$LONGINT$BYTE ; 
0x080002
Result := pxBuffer;
  08000980   LDR            R0, [R11, #-0x30]
  08000984   STR            R0, [R11, #-0x3C]

Am 21.04.20 um 23:07 schrieb Joao Schuler:
> just as point for consideration, I'm not sure if data alignment will 
> improve speed on future processors:
> https://lemire.me/blog/2012/05/31/data-alignment-for-speed-myth-or-reality/ 
>
>
> Food for thought: imagine if we had single (32 bits floating point) 
> values dynamic arrays with 1 million values each: a b and c. I would 
> love to have something like this:
> a := b + c;
>
> _______________________________________________
> fpc-devel maillist  -  fpc-devel at lists.freepascal.org
> https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
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