[PATCH] D73319: Add extension 'gnu_asm_goto_with_outputs'

[Bill Wendling via Phabricator via cfe-commits]

void updated this revision to Diff 240681.
void added a comment.

Add description of "gnu_asm_goto_with_outputs" to the language extensions doc.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D73319/new/

https://reviews.llvm.org/D73319

Files:
  clang/docs/LanguageExtensions.rst
  clang/include/clang/Basic/Features.def
  clang/test/Parser/asm-goto.c


Index: clang/test/Parser/asm-goto.c
===================================================================
--- clang/test/Parser/asm-goto.c
+++ clang/test/Parser/asm-goto.c
@@ -4,6 +4,9 @@
 #if !__has_extension(gnu_asm)
 #error Extension 'gnu_asm' should be available by default
 #endif
+#if !__has_extension(gnu_asm_goto_with_outputs)
+#error Extension 'gnu_asm_goto_with_outputs' should be available by default
+#endif
 
 
 int a, b, c, d, e, f, g, h, i, j, k, l;
Index: clang/include/clang/Basic/Features.def
===================================================================
--- clang/include/clang/Basic/Features.def
+++ clang/include/clang/Basic/Features.def
@@ -252,6 +252,7 @@
 EXTENSION(pragma_clang_attribute_namespaces, true)
 EXTENSION(pragma_clang_attribute_external_declaration, true)
 EXTENSION(gnu_asm, LangOpts.GNUAsm)
+EXTENSION(gnu_asm_goto_with_outputs, LangOpts.GNUAsm)
 
 #undef EXTENSION
 #undef FEATURE
Index: clang/docs/LanguageExtensions.rst
===================================================================
--- clang/docs/LanguageExtensions.rst
+++ clang/docs/LanguageExtensions.rst
@@ -1255,6 +1255,34 @@
 
 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.
+
+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 constriants:
+
+.. code-block:: c++
+
+  int foo(int x) {
+      int y;
+      asm goto("# %0 %1 %2" : "=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.
+
+Query for this feature with ``__has_extension(gnu_asm_goto_with_outputs)``.
+
 Objective-C Features
 ====================
 


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