https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81484
Bug ID: 81484
Summary: incorrect -Wint-in-bool-context warning
Product: gcc
Version: 7.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: arnd at linaro dot org
Target Milestone: ---
I came across a warning in the linux kernel with gcc-7.1.1:
arch/x86/math-emu/reg_add_sub.c: In function 'FPU_add':
arch/x86/math-emu/reg_add_sub.c:80:48: error: ?: using integer constants in
boolean context [-Werror=int-in-bool-context]
After analyzing it further, I reduced it to this test program, please compile
with "gcc-7.1.1 -Wall -O2":
#define signbyte(a) (((unsigned char *)(a))[9])
#define getsign(a) (signbyte(a) & 0x80)
#define setsign(a,b) do { if (b) signbyte(a) |= 0x80; else signbyte(a) &=
0x7f; } while (0)
#define CW_RC 0x0C00
#define RC_DOWN 0x0400
#define SIGN_POS 0u
#define SIGN_NEG 0x80u
int FPU_add(void *dest, int control_w)
{
/* a) correct code, bogus warning */
setsign(dest, ((control_w & CW_RC) != RC_DOWN) ? SIGN_POS : SIGN_NEG);
/* b) false-positive warning */
setsign(dest, (control_w & CW_RC) != RC_DOWN ? SIGN_POS : SIGN_NEG);
/* c) wrong code, no warning */
setsign(dest, (control_w & CW_RC) != (RC_DOWN ? SIGN_POS : SIGN_NEG));
return 0;
}
The warning we get is about case a), which as far as I can tell is written
correctly and unambiguous, and should not trigger a warning.
It appears that the compiler mistakes this for code that is written in a
somewhat ambiguous way like the (still-correct) variant b) when the author
actually meant to write it like c). The result is that we also get a
false-positive warning for b) that makes a lot of sense, while the incorrect
case c) is a false-negative here, as we don't get a warning.
