[clang] 8c7b346 - [clang] Update ASM goto documentation to reflect how Clang differs from GCC

[Bill Wendling via cfe-commits]

Author: Bill Wendling
Date: 2022-10-13T12:05:40-07:00
New Revision: 8c7b3461a5346a66088d884500c2b3c2829b4652

URL: https://github.com/llvm/llvm-project/commit/8c7b3461a5346a66088d884500c2b3c2829b4652
DIFF: https://github.com/llvm/llvm-project/commit/8c7b3461a5346a66088d884500c2b3c2829b4652.diff

LOG: [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

Added: 
    

Modified: 
    clang/docs/LanguageExtensions.rst

Removed: 
    


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diff  --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
index b449809c88d5..fca4227a5812 100644
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -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 
diff ers 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
+  
diff erent 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