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Subject: [Bug c++/101649] New: -Wdouble-promotion warning emitted when
 floating point literals are not actually promoted to doubles
Date: Tue, 27 Jul 2021 18:38:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101649

            Bug ID: 101649
           Summary: -Wdouble-promotion warning emitted when floating point
                    literals are not actually promoted to doubles
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: make_all@t-online.de
  Target Milestone: ---

GCC emits a double promotion warning when using the literal 0.5 with a floa=
t.
For example

float func(float num) {
    return num * 0.5;
}

warning: implicit conversion from 'float' to 'double' to match other operan=
d of
binary expression [-Wdouble-promotion]
    3 |     return num * 0.5;

However, GCC does not actually use the double representation for this liter=
al
(and others than can be represented equally in float as in double).
The assembler output for both "0.5" and "0.5f" is the same:

func(float):
        push    rbp
        mov     rbp, rsp
        movss   DWORD PTR [rbp-4], xmm0
        movss   xmm1, DWORD PTR [rbp-4]
        movss   xmm0, DWORD PTR .LC0[rip]
        mulss   xmm0, xmm1
        pop     rbp
        ret
.LC0:
        .long   1056964608

This warning is misleading and should not be emitted=