[clang] [clang] Implement -fstrict-bool (PR #160790)

[John McCall via cfe-commits]

=?utf-8?q?Félix?= Cloutier <fcloutier at apple.com>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/160790 at github.com>


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@@ -2306,6 +2306,24 @@ are listed below.
    additional function arity information (for supported targets). See
    :doc:`ControlFlowIntegrity` for more details.
 
+.. option:: -fstrict-bool
+
+    ``bool`` values are stored to memory as 8-bit values on most targets. Under
+    ``-fstrict-bool``, it is undefined behavior for a ``bool`` value stored in
+    memory to have any other bit pattern than 0 or 1. This creates some
+    optimization opportunities for the compiler, but it enables memory
+    corruption if that assumption is violated, for instance if any other value
+    is ``memcpy``ed over a ``bool``. This is enabled by default.
+
+.. option:: -fno-strict-bool[={truncate|nonzero}]
+
+    Disable optimizations based on the assumption that all ``bool`` values,
+    which are typically represented as 8-bit integers in memory, only ever
+    contain bit patterns 0 or 1. When =truncate is specified, a ``bool`` is
+    true if its least significant bit is set, and false otherwise. When =nonzero
+    is specified, a ``bool`` is true when any bit is set, and false otherwise.
+    The default is =truncate.
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rjmccall wrote:

Please put the `=` strings in backticks.

Might be worth saying, just to be perfectly clear, that Clang always stores either 0 or 1 regardless of the value of this setting.

https://github.com/llvm/llvm-project/pull/160790