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Deprecate Q_ASSUME

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No replacement provided because we didn't agree on how to do it. But the
current implementation is bad (with GCC, at least), so tell people to
stop using it.

[ChangeLog][Deprecations] The Q_ASSUME macro is now deprecated. Do not
use it in new code and consider removing it from existing code.

Change-Id: Idd5e1bb52be047d7b4fffffd175322ee3402756b
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
This commit is contained in:
Thiago Macieira 2023-04-05 17:11:32 -03:00
parent c98d70ef7b
commit 275e0e48a9
2 changed files with 11 additions and 20 deletions
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29
src/corelib/global/qassert.cpp
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@ -139,29 +139,20 @@ void qBadAlloc()
/*!
\macro void Q_ASSUME(bool expr)
\deprecated
\relates <QtAssert>
\since 5.0
Causes the compiler to assume that \a expr is \c true. This macro is useful
for improving code generation, by providing the compiler with hints about
conditions that it would not otherwise know about. However, there is no
guarantee that the compiler will actually use those hints.
Causes the compiler to assume that \a expr is \c true.
This macro could be considered a "lighter" version of \l{Q_ASSERT()}. While
Q_ASSERT will abort the program's execution if the condition is \c false,
Q_ASSUME will tell the compiler not to generate code for those conditions.
Therefore, it is important that the assumptions always hold, otherwise
undefined behavior may occur.
This macro is known to produce worse code than when no assumption was
inserted in the code, with some compiler versions. The arguments passed to
it are always evaluated, even in release mode, with some compilers and not
others, so application code needs to be aware of those possible differences
in behavior.
If \a expr is a constantly \c false condition, Q_ASSUME will tell the compiler
that the current code execution cannot be reached. That is, Q_ASSUME(false)
is equivalent to Q_UNREACHABLE().
In debug builds the condition is enforced by an assert to facilitate debugging.
\note Q_LIKELY() tells the compiler that the expression is likely, but not
the only possibility. Q_ASSUME tells the compiler that it is the only
possibility.
Do not use it in new code. It is retained as-is for compatibility with old
code and will likely be removed in the next major version Qt.
\sa Q_ASSERT(), Q_UNREACHABLE(), Q_LIKELY()
*/
@ -199,7 +190,7 @@ void qBadAlloc()
compilers that need them, without causing warnings for compilers that
complain about its presence.
\sa Q_ASSERT(), Q_ASSUME(), qFatal(), Q_UNREACHABLE_RETURN()
\sa Q_ASSERT(), qFatal(), Q_UNREACHABLE_RETURN()
*/
/*!

2
src/corelib/global/qcompilerdetection.qdoc
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@ -197,7 +197,7 @@
\snippet code/src_corelib_global_qglobal.cpp qlikely
\sa Q_UNLIKELY(), Q_ASSUME()
\sa Q_UNLIKELY()
*/
/*!