[Bug c/91312] -Wconversion warning with += operator
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91312 Jason Merrill changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED CC||jason at gcc dot gnu.org Resolution|--- |DUPLICATE --- Comment #9 from Jason Merrill --- This is fixed by the patch for 40752. *** This bug has been marked as a duplicate of bug 40752 ***
[Bug c/91312] -Wconversion warning with += operator
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91312 --- Comment #8 from Marc Glisse --- We know that the warning is not so useful as is, that's why it isn't part of Wall or Wextra, see the other bugs on the topic. It needs people with time and motivation to work on it.
[Bug c/91312] -Wconversion warning with += operator
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91312 --- Comment #7 from Kostas Sotiropoulos --- (In reply to Andrew Pinski from comment #5) > (In reply to Kostas Sotiropoulos from comment #4) > > Any comments on my questions? > > Yes go read the c standard about prompting to int here. I had checked the standard from the following link: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf I suppose that you mean the following two sections that cover our case: 6.5.16.2 A compound assignment of the form E1 op=E2 is equivalent to the simple assignment expression E1 = E1 op (E2), except that the lvalue E1 is evaluated only once, and with respect to an indeterminately-sequenced function call, the operation of a compound assignment is a single evaluation. 6.3.1.1 Boolean, characters, and integers 2 The following may be used in an expression wherever an int or unsigned int may be used: — An object or expression with an integer type (other than int or unsigned int) whose integer conversion rank is less than or equal to the rank of int and unsigned int. — A bit-field of type _Bool, int, signed int, or unsigned int. If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions. All other types are unchanged by the integer promotions. So considering the above two sections, if we even cast i += (unsigned char) MACRO then this would be equal to i = i + (unsigned char) MACRO. But, what if I want to avoid this warning then the only way to get rid of it is the following way? i = (unsigned char) (i + MACRO). But we will lose on code optimization be this way, do not we? Especially when we are talking about Embedded Systems software. Please note that the same code snippet passes through clang compiler (clang version 3.8.1-24) without any warning when -Werror=conversion is used during compilation. I suppose that even this compiler follows C standard. In my opinion such kind of warnings are false positives that should not be reported from gcc compiler too. Maybe a change of this kind on gcc compiler opens the Pandora's Box for other things but someone should not avoid of defining MACROS and use them on compound statements (+=, -=, *= etc) and sacrifice code optimization by expanding them. Waiting for your comments.
[Bug c/91312] -Wconversion warning with += operator
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91312 Eric Gallager changed: What|Removed |Added CC||egallager at gcc dot gnu.org --- Comment #6 from Eric Gallager --- (In reply to Andrew Pinski from comment #2) > THis is not a bug, In C, "i += MACRO;" is equivant to: > i = i + MACRO. > And since you are using a type smaller than int, it is prompted to int. > > NOTE there might be another bug associated with this one. bug 40752?
[Bug c/91312] -Wconversion warning with += operator
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91312 --- Comment #5 from Andrew Pinski --- (In reply to Kostas Sotiropoulos from comment #4) > Any comments on my questions? Yes go read the c standard about prompting to int here.
[Bug c/91312] -Wconversion warning with += operator
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91312 --- Comment #4 from Kostas Sotiropoulos --- Any comments on my questions?
[Bug c/91312] -Wconversion warning with += operator
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91312 --- Comment #3 from Kostas Sotiropoulos --- (In reply to Andrew Pinski from comment #2) > THis is not a bug, In C, "i += MACRO;" is equivant to: > i = i + MACRO. > And since you are using a type smaller than int, it is prompted to int. > > NOTE there might be another bug associated with this one. 1)In order to remove the warning we should only do the following: i = (unsigned char)(i + MACRO)? 2)Why if we want to cast like this: i += (unsigned char)MACRO, i on the right side of expression is not also casted to unsigned char (I mean the second i of expanded expression i = i + MACRO? 3)What do you mean with your NOTE can you please elaborate a bit more? 4) If we expand and cast the expression we will probably have not so optimized produced code by the compiler, should we? Thank you, Kostas
[Bug c/91312] -Wconversion warning with += operator
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91312 Andrew Pinski changed: What|Removed |Added Keywords||diagnostic --- Comment #2 from Andrew Pinski --- THis is not a bug, In C, "i += MACRO;" is equivant to: i = i + MACRO. And since you are using a type smaller than int, it is prompted to int. NOTE there might be another bug associated with this one.
[Bug c/91312] -Wconversion warning with += operator
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91312
--- Comment #1 from Kostas Sotiropoulos ---
Hi,
When compiling the following code snippet with gcc 8.3.0
with -Werror=conversion option:
#include
#define MACRO 1
int main(void)
{
unsigned char i;
i += MACRO;
return i;
}
the following warning which is treated as an error is reported:
test.c:3:15: error: conversion from ‘int’ to ‘unsigned char’ may change value
[-Werror=conversion]
#define MACRO 1
^
test.c:9:7: note: in expansion of macro ‘MACRO’
i += MACRO;
^
cc1: some warnings being treated as errors
Should this happen?
From what is described about -Wconversion under the following link:
https://gcc.gnu.org/onlinedocs/gcc-8.3.0/gcc.pdf I am not convinced
that even with casting the right hand of expression this problem should
still appear on this case. Please check this statement:
"Do not warn for explicit casts likeabs ((int) x)andui= (unsigned) -1,
or if the value is not changed by the conversion like inabs(2.0)."
The only solution to this issue is to expand += and then cast to unsigned
char i.e. i = (unsigned char) (i + MACRO)? Should not we be more flexible
on cases of MACRO's even here that integer promotion takes place?
Thank you,
Kostas
