Hi,

I was reading this article about Delphi XE2 floating point performance.

http://delphitools.info/2011/09/02/first-look-at-xe2-floating-point-performance/

Not that I understand much of the assembler generated, but what I did notice is that Delphi XE2 64-bit uses the MOVAPD function (introduced in SSE2 [1]), but even if I specify -O3 -CfSSE3 with 64-bit FPC, FPC only uses the MOVSD (introduced in 386 [2]).

So is there place for optimizing FPC a bit more? Reducing the number of instructions and using faster / newer assembler calls?

What does the compiler generate for the two following lines?

  x := x0 * x0 - y0 * y0 + p;
  y := 2 * x0 * y0 + q;

Delphi XE2
-----------
FMandelTest.pas.193: x := x0 * x0 - y0 * y0 + p;
00000000005A1452 660F28C4 movapd xmm0,xmm4 00000000005A1456 F20F59C4 mulsd xmm0,xmm4
00000000005A145A 660F28CD movapd xmm1,xmm5 00000000005A145E F20F59CD mulsd xmm1,xmm5
00000000005A1462 F20F5CC1 subsd xmm0,xmm1
00000000005A1466 F20F58C2 addsd xmm0,xmm2
FMandelTest.pas.194: y := 2 * x0 * y0 + q;
00000000005A146A 660F28CC movapd xmm1,xmm4 00000000005A146E F20F590DA2000000 mulsd xmm1,qword ptr [rel $000000a2] 00000000005A1476 F20F59CD mulsd xmm1,xmm5
00000000005A147A F20F58CB addsd xmm1,xmm3

64-bit FPC 2.5.1
-----------------
# Var x located in register xmm0
# Var x0 located in register xmm2
# Var y located in register xmm0
# Var y0 located in register xmm3
# Var p located in register xmm4
# Var q located in register xmm8
......
.Ll3:
# [17] x := x0 * x0 - y0 * y0 + p;
movsd %xmm0,%xmm5
mulsd %xmm0,%xmm5
.Ll4:
movsd %xmm3,%xmm1
.Ll5:
movsd %xmm1,%xmm0
mulsd %xmm1,%xmm0
subsd %xmm0,%xmm5
addsd %xmm4,%xmm5
movsd %xmm5,-40(%rbp)
.Ll6:
# [18] y := 2 * x0 * y0 + q;
movsd _$FPU_TEST$_Ld1,%xmm0
mulsd %xmm2,%xmm0
mulsd %xmm3,%xmm0
addsd %xmm8,%xmm0
movsd %xmm0,-32(%rbp)

References:
===========
1)
http://en.wikipedia.org/wiki/MOVAPD
2)
http://en.wikipedia.org/wiki/X86_instruction_listings#Added_with_80386

The full Delphi source code for the Mandelbrot test can be downloaded from:

http://delphitools.info/wp-content/uploads/2011/03/MandelTest.zip

Regards,
- Graeme -

Graeme Geldenhuys wrote

Not that I understand much of the assembler generated, but what I did notice is that Delphi XE2 64-bit uses the MOVAPD function (introduced in SSE2 [1]), but even if I specify -O3 -CfSSE3 with 64-bit FPC, FPC only uses the MOVSD (introduced in 386 [2]).

The movsd instruction you are thinking of is something completely different (and would appear as "movsl" in the assembler code, because it uses AT&T syntax). The movsd that FPC uses is an SSE2 instruction.

Jonas

19.10.2011 17:45, Graeme Geldenhuys пишет:

There are actually two different instructions called MOVSD, one for string manipulations, another one is SSE2. Another such ambiguous instruction is CMPSD. This causes some confusion, and the wiki page [2] contains a note on that.
The MOVSD instruction used by FPC is the from the SSE2 family, as well as MOVAPD, and has nothing to do with 386.

Regards,
Sergei

Jonas Maebe wrote

The movsd that FPC uses is an SSE2 instruction.

Ah, thanks for the info.

Regards,
- Graeme -