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Repository: rG LLVM Github Monorepo https://reviews.llvm.org/D135818 Files: clang/docs/LanguageExtensions.rst Index: clang/docs/LanguageExtensions.rst =================================================================== --- clang/docs/LanguageExtensions.rst +++ clang/docs/LanguageExtensions.rst @@ -1546,28 +1546,13 @@ ASM Goto with Output Constraints ================================ -In addition to the functionality provided by `GCC's extended -assembly <https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html>`_, clang -supports output constraints with the `goto` form. +.. note:: -The goto form of GCC's extended assembly allows the programmer to branch to a C -label from within an inline assembly block. Clang extends this behavior by -allowing the programmer to use output constraints: - -.. code-block:: c++ - - int foo(int x) { - int y; - asm goto("# %0 %1 %l2" : "=r"(y) : "r"(x) : : err); - return y; - err: - return -1; - } - -It's important to note that outputs are valid only on the "fallthrough" branch. -Using outputs on an indirect branch may result in undefined behavior. For -example, in the function above, use of the value assigned to `y` in the `err` -block is undefined behavior. + Clang's implementation of ASM goto differs from `GCC's + implementation<https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html>`_ in + that Clang doesn't support outputs on the indirect branch. Use of an output + on the indirect branch may result in undefined behavior and should be + avoided. When using tied-outputs (i.e. outputs that are inputs and outputs, not just outputs) with the `+r` constraint, there is a hidden input that's created
Index: clang/docs/LanguageExtensions.rst =================================================================== --- clang/docs/LanguageExtensions.rst +++ clang/docs/LanguageExtensions.rst @@ -1546,28 +1546,13 @@ ASM Goto with Output Constraints ================================ -In addition to the functionality provided by `GCC's extended -assembly <https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html>`_, clang -supports output constraints with the `goto` form. +.. note:: -The goto form of GCC's extended assembly allows the programmer to branch to a C -label from within an inline assembly block. Clang extends this behavior by -allowing the programmer to use output constraints: - -.. code-block:: c++ - - int foo(int x) { - int y; - asm goto("# %0 %1 %l2" : "=r"(y) : "r"(x) : : err); - return y; - err: - return -1; - } - -It's important to note that outputs are valid only on the "fallthrough" branch. -Using outputs on an indirect branch may result in undefined behavior. For -example, in the function above, use of the value assigned to `y` in the `err` -block is undefined behavior. + Clang's implementation of ASM goto differs from `GCC's + implementation<https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html>`_ in + that Clang doesn't support outputs on the indirect branch. Use of an output + on the indirect branch may result in undefined behavior and should be + avoided. When using tied-outputs (i.e. outputs that are inputs and outputs, not just outputs) with the `+r` constraint, there is a hidden input that's created
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