[PATCH] D135818: [clang] Update ASM goto documentation to reflect how Clang differs from GCC

[Bill Wendling via Phabricator via cfe-commits]

void updated this revision to Diff 467284.
void added a comment.

Update with example.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D135818/new/

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 +1546,23 @@
 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 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++
-
-  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


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