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[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
This commit is contained in:
Bill Wendling 2022-10-12 14:43:38 -07:00
parent 02a543db66
commit 8c7b3461a5
1 changed files with 14 additions and 17 deletions
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31
clang/docs/LanguageExtensions.rst
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@ -1546,28 +1546,25 @@ Query for this feature with ``__has_extension(blocks)``.
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:
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 yet support outputs on the indirect branch. Use of an
output on the indirect branch may result in undefined behavior and should be
avoided. E.g., in the following `z` isn't valid when used and may have
different values depending on optimization levels. (Clang may not warn about
such constructs.)
.. code-block:: c++
.. code-block:: c++
int foo(int x) {
int y;
asm goto("# %0 %1 %l2" : "=r"(y) : "r"(x) : : err);
int foo(int x) {
int y, z;
asm goto(... : "=r"(y), "=r"(z): "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.
return z;
}
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