• d-bugmail puremagic.com (60/60) Sep 23 2017 https://issues.dlang.org/show_bug.cgi?id=17854

d-bugmail puremagic.com writes:

https://issues.dlang.org/show_bug.cgi?id=17854

          Issue ID: 17854
           Summary: Suboptimal code generated with constants and SSE
           Product: D
           Version: D2
          Hardware: x86_64
                OS: All
            Status: NEW
          Keywords: performance
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: b2.temp gmx.com

in both case with -O -release

A. FP constant
==============

int test(float i)
{
    return cast(int)(2.0 * i * i);
}

disassembles to:

000000000045B640h  push rbp
000000000045B641h  mov rbp, rsp
000000000045B644h  sub rsp, 10h
000000000045B648h  movss xmm3, xmm0
000000000045B64Ch  cvtss2sd xmm1, xmm0
000000000045B650h  mov rax, 4000000000000000h
000000000045B65Ah  mov qword ptr [rbp-10h], rax
000000000045B65Eh  movsd xmm2, qword ptr [rbp-10h]
000000000045B663h  mulsd xmm1, xmm2
000000000045B667h  cvtss2sd xmm4, xmm0
000000000045B66Bh  mulsd xmm1, xmm4
000000000045B66Fh  cvttsd2si eax, xmm1
000000000045B673h  mov rsp, rbp
000000000045B676h  pop rbp
000000000045B677h  ret 


B. Integer constant
===================

int test(float i)
{
    return cast(int)(2 * i * i);
}

disassembles to:

000000000045B640h  push rbp
000000000045B641h  mov rbp, rsp
000000000045B644h  sub rsp, 10h
000000000045B648h  movss xmm2, xmm0
000000000045B64Ch  mov eax, 40000000h
000000000045B651h  mov dword ptr [rbp-10h], eax
000000000045B654h  movss xmm1, dword ptr [rbp-10h]
000000000045B659h  mulss xmm0, xmm1
000000000045B65Dh  mulss xmm0, xmm2
000000000045B661h  cvttss2si eax, xmm0
000000000045B665h  mov rsp, rbp
000000000045B668h  pop rbp
000000000045B669h  ret 


case A could clearly be compiled as B since the fractional part of the constant
is empty.

--