forked from OSchip/llvm-project
[clang] Update ASM goto documentation to reflect how Clang differs from GCC
That said, we are planning to add this support in the near future. Link: https://github.com/llvm/llvm-project/issues/53562 Differential Revision: https://reviews.llvm.org/D135818
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@ -1546,29 +1546,26 @@ Query for this feature with ``__has_extension(blocks)``.
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ASM Goto with Output Constraints
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================================
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In addition to the functionality provided by `GCC's extended
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assembly <https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html>`_, clang
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supports output constraints with the `goto` form.
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.. note::
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The goto form of GCC's extended assembly allows the programmer to branch to a C
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label from within an inline assembly block. Clang extends this behavior by
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allowing the programmer to use output constraints:
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Clang's implementation of ASM goto differs from `GCC's
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implementation<https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html>`_ in
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that Clang doesn't yet support outputs on the indirect branch. Use of an
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output on the indirect branch may result in undefined behavior and should be
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avoided. E.g., in the following `z` isn't valid when used and may have
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different values depending on optimization levels. (Clang may not warn about
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such constructs.)
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.. code-block:: c++
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int foo(int x) {
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int y;
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asm goto("# %0 %1 %l2" : "=r"(y) : "r"(x) : : err);
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int y, z;
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asm goto(... : "=r"(y), "=r"(z): "r"(x) : : err);
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return y;
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err:
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return -1;
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return z;
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}
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It's important to note that outputs are valid only on the "fallthrough" branch.
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Using outputs on an indirect branch may result in undefined behavior. For
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example, in the function above, use of the value assigned to `y` in the `err`
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block is undefined behavior.
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When using tied-outputs (i.e. outputs that are inputs and outputs, not just
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outputs) with the `+r` constraint, there is a hidden input that's created
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before the label, so numeric references to operands must account for that.
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