linux-kernel - [PATCH 3/8] x86: math-emu: avoid bogus -Wint-in-bool-context warning

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Date:   Wed, 19 Jul 2017 14:53:01 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     x86@...nel.org, Bill Metzenthen <billm@...bpc.org.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: [PATCH 3/8] x86: math-emu: avoid bogus -Wint-in-bool-context warning

gcc-7.1.1 produces this warning:

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]

This appears to be a bug in gcc-7.1.1, and I have reported it as
PR81484. The compiler suggests that code written as

	if (a & b ? c : d)

is usually incorrect and should have been

	if (a & (b ? c : d))

However, in this case, we correctly write

	if ((a & b) ? c : d)

and should not get a warning for it.

This adds a dirty workaround for the problem, adding a comparison with
zero inside of the macro. The warning is currently disabled in the kernel,
so we may decide not to apply the patch, and instead wait for future gcc
releases to fix the problem. On the other hand, it seems to be the
only instance of this particular problem.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81484
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
Originally sent on July 14, this is the same patch again with
an rewritten changelog.
---
 arch/x86/math-emu/fpu_emu.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/math-emu/fpu_emu.h b/arch/x86/math-emu/fpu_emu.h
index afbc4d805d66..c9c320dccca1 100644
--- a/arch/x86/math-emu/fpu_emu.h
+++ b/arch/x86/math-emu/fpu_emu.h
@@ -157,7 +157,7 @@ extern u_char const data_sizes_16[32];
 
 #define signbyte(a) (((u_char *)(a))[9])
 #define getsign(a) (signbyte(a) & 0x80)
-#define setsign(a,b) { if (b) signbyte(a) |= 0x80; else signbyte(a) &= 0x7f; }
+#define setsign(a,b) { if ((b) != 0) signbyte(a) |= 0x80; else signbyte(a) &= 0x7f; }
 #define copysign(a,b) { if (getsign(a)) signbyte(b) |= 0x80; \
                         else signbyte(b) &= 0x7f; }
 #define changesign(a) { signbyte(a) ^= 0x80; }
-- 
2.9.0