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

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=?utf-8?q?Félix?= Cloutier <fcloutier at apple.com>,
=?utf-8?q?Félix?= Cloutier <fcloutier at apple.com>,
=?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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@@ -2312,6 +2312,30 @@ 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. C and
+    C++ specify that it is undefined behavior to put a value other than 0 or 1
+    in the storage of a ``bool`` value, and with ``-fstrict-bool``, Clang
+    leverages this knowledge for optimization opportunities. When this
+    assumption is violated, for instance if invalid data is ``memcpy``ed over a
+    ``bool``, the optimized code can lead to memory corruption.
+    ``-fstrict-bool`` 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``, but this could change in
+    future releases.
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apple-fcloutier wrote:

I think that constancy would be a good reason to set the default in stone. This PR's been open for months and I avoided making changes there until we settled on a solution.

With that said, I don't think we can say this is an ABI break without implying that code built with `-fstrict-bool`, `-fno-strict-bool=truncate` and `-fno-strict-bool=nonzero` are each mutually ABI-incompatible. Since that outcome is incompatible with the flag's expected usage (strategic deployment in programs that need to be protected from mistakes around the handling of bool values), we need to conclude that sharing broken bools across an ABI boundary is still undefined.

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