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Use Clang 3.6's __builtin_assume in Q_ASSUME 

It's more efficient than the current implementation. Example:

int f(int i)
{
    Q_ASSUME(i < 8);
    return i < 8;
}

Before:
        cmpl    $8, %edi
        setl    %al
        movzbl  %al, %eax
        retq

After:
        movl    $1, %eax
        retq

Change-Id: I1a800c709d3543699131ffff13c1c50713a8da2c
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
bb10
Thiago Macieira 2015-02-10 22:19:51 -08:00
parent 738e9b185c
commit b584648df1
1 changed files with 5 additions and 1 deletions
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6
src/corelib/global/qcompilerdetection.h
View File

@ -173,7 +173,11 @@
# else
# define Q_CC_CLANG ((__clang_major__ * 100) + __clang_minor__)
# endif
# define Q_ASSUME_IMPL(expr) if (expr){} else __builtin_unreachable()
# if __has_builtin(__builtin_assume)
# define Q_ASSUME_IMPL(expr) __builtin_assume(expr)
# else
# define Q_ASSUME_IMPL(expr) if (expr){} else __builtin_unreachable()
# endif
# define Q_UNREACHABLE_IMPL() __builtin_unreachable()
# if !defined(__has_extension)
# /* Compatibility with older Clang versions */