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Re: F>R, FR@ and FR> Float to return stack words

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On Sun, 30 Aug 2026 13:31:56 GMT
anton@mips.complang.tuwien.ac.at (Anton Ertl) wrote:

> peter <peter.noreply@tin.it> writes:
> >If I do the school book formual I get
> >
> >: Z*  {f: x y u v :} 
> >	x u f* y v f* f- x v f* y u f* f+ ;
> >
> >
> >Which give more compact code
> >
> >seea Z*
> >0x42A730  C4C17B104D00         vmovsd xmm1, qword ptr [r13]
> >0x42A736  C4C173594D10         vmulsd xmm1, xmm1, qword ptr [r13+0x10]
> >0x42A73C  C4C17B595508         vmulsd xmm2, xmm0, qword ptr [r13+0x8]
> >0x42A742  C5F35CCA             vsubsd xmm1, xmm1, xmm2
> >0x42A746  C4C17B595510         vmulsd xmm2, xmm0, qword ptr [r13+0x10]
> >0x42A74C  C4C17B105D00         vmovsd xmm3, qword ptr [r13]
> >0x42A752  C4C163595D08         vmulsd xmm3, xmm3, qword ptr [r13+0x8]
> >0x42A758  C5E358DA             vaddsd xmm3, xmm3, xmm2
> >0x42A75C  C5E310C3             vmovsd xmm0, xmm3, xmm3
> >0x42A760  C4C17B114D10         vmovsd qword ptr [r13+0x10], xmm1
> >0x42A766  4D8D6D10             lea r13, [r13+0x10]
> >0x42A76A  C3                   ret
> >59 bytes, 12 instructions
> 
> Cool!
> 
> >I tried to measure the execution times. This shows no difference!
> 
> Which indicates that the bottleneck of your benchmark is not in the z*
> code.
> 
> >I had to start with the complex number already on the stack!
> >If I put fpi 345e4 inside the loop elapsed time increase with 100 times!
> 
> How do you implement FP literals?  One technique is to call FLIT, have
> the FP literal behind the call, and let FLIT load from its return
> address, then change its return address.  That results in a branch
> misprediction (~20 cycles) for every FLIT, but even 2 FLITs with this
> technique do not explain 100* slowdown.

I found the main problem. It was FPI that was implemented as a call
to the C math library I use. It goes thru several calls saves all regs
and switches the stack to the original (correctly aligned).

Now it is defined as 

$4009´21FB´5444´2D18 pad ! pad f@ FCONSTANT FPI

This compiles to the token code

see fpi macro
Address   OP                         Instruction
0x720300  81 182D4454FB210940        FLIT  0x400921FB54442D18
0x720309  25                         RET
10 bytes, 2 instructions

It is a macro so will get inlined
The assembler code becomes
seea fpi
0x423AD0  48B8182D4454FB210940 mov rax, 0x400921FB54442D18
0x423ADA  C4E1F96EC8           vmovq xmm1, rax
0x423ADF  C4C17B1145F8         vmovsd qword ptr [r13-0x8], xmm0
0x423AE5  C5F310C1             vmovsd xmm0, xmm1, xmm1
0x423AE9  4D8D6DF8             lea r13, [r13-0x8]
0x423AED  C3                   ret
30 bytes, 6 instructions
ok

2 instruction to load the litteral in a xmm register
It is a bit long but I do not think it can be better

The other possible solution would be to store litterals in memmory
and load from there. I think this is  what Aarch64 compilers do

Peter
 
> - anton

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F>R, FR@ and FR> Float to return stack words peter <peter.noreply@tin.it> - 2026-08-28 00:04 +0200
  Re: F>R, FR@ and FR> Float to return stack words dxf <dxforth@gmail.com> - 2026-08-28 14:44 +1000
  Re: F>R, FR@ and FR> Float to return stack words anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2026-08-28 06:50 +0000
  Re: F>R, FR@ and FR> Float to return stack words marcel hendrix <mhx@iae.nl> - 2026-08-28 19:35 +0200
  Re: F>R, FR@ and FR> Float to return stack words peter <peter.noreply@tin.it> - 2026-08-30 10:06 +0200
    Re: F>R, FR@ and FR> Float to return stack words anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2026-08-30 13:31 +0000
      Re: F>R, FR@ and FR> Float to return stack words peter <peter.noreply@tin.it> - 2026-08-30 15:58 +0200
        Re: F>R, FR@ and FR> Float to return stack words anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2026-08-30 15:12 +0000

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