[Bug c++/101649] New: -Wdouble-promotion warning emitted when floating point literals are not actually promoted to doubles
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101649
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_...@t-online.de
Target Milestone: ---
GCC emits a double promotion warning when using the literal 0.5 with a float.
For example
float func(float num) {
return num * 0.5;
}
warning: implicit conversion from 'float' to 'double' to match other operand of
binary expression [-Wdouble-promotion]
3 | return num * 0.5;
However, GCC does not actually use the double representation for this literal
(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):
pushrbp
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
[Bug c++/101649] -Wdouble-promotion warning emitted when floating point literals are not actually promoted to doubles
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101649 --- Comment #2 from make_...@t-online.de --- I don't want to avoid double promotion as double promotion is not actually happening. I want to not be bothered by a false warning :)
[Bug c++/101649] -Wdouble-promotion warning emitted when floating point literals are not actually promoted to doubles
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101649 --- Comment #4 from South Window --- For the user of GCC it doesn't matter whether GCC at compile times converts the literal to a double, and then notices that 32 bits of are zero, and uses a float instead, or if GCC knows right away that 0.5 (and others) don't need to be promoted. Promoting first and then completely undoing the promotion is a purely internal process, effectively a null operation that is of no interest of the user of the compiler. One of the most relevant scopes for -Wdouble-promotion is probably optimization (in particular for SIMD), where the unintended use of a double would cause a significant performance hit. But if the compiler is smart enough and is not using doubles and double instructions, and no precision is changed anywhere in the process, it should be smart enough to not through a this warning.
[Bug c++/101649] -Wdouble-promotion warning emitted when floating point literals are not actually promoted to doubles
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101649 --- Comment #6 from South Window --- Not doing this optimization in -O0 would still leave this warning wrong when optimization is turned on. In my view, the fundamental difference between a generic linter and compiler warnings is that the compiler warning should reflect what the compiler is really doing. If it is not easy to not emit this warning, could it be possible to undo this warning from "elsewhere", or at least to accompany this warning with a withdrawal, e.g. warning: implicit conversion from 'float' to 'double' to match other operand of binary expression [-Wdouble-promotion] warning withdrawn: literal '0.5' can be represented without loss of precision as 'float'. Therefore, 'float' precision is used instead of 'double'. [-Wdouble-promotion]
[Bug c++/101649] -Wdouble-promotion warning emitted when floating point literals are not actually promoted to doubles
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101649 --- Comment #7 from South Window --- Since the code generation seems to always reduce a double literal to a floating point in the cases this discussed here, could the function that throws this warning be extended to not throw the warning if all 32 least significant bits of the double literal are zero?
