[LangRef] update according to unwinding support in inline asm

[cynecx]

Summary

https://reviews.llvm.org/D95745 introduced a new unwind keyword for inline assembler expressions. Inline asms marked with the unwind keyword allows stack unwinding from inline assembly because the compiler emits unwinding information ("around" the inline asm) as it would for calls/invokes. Unwinding the stack from within non-unwind inline asm may cause UB.

[jdoerfert]

I think more commit message text is necessary here.

Would this mean asm w/o unwind cannot unwind?

[cynecx]

I think more commit message text is necessary here.

I agree.

Would this mean asm w/o unwind cannot unwind?

The unwind keyword causes llvm to treat the inline assembly as "unwindable", so appropriate unwind information/tables are emitted by codegen. By omitting the unwind keyword, llvm doesn't emit these information and therefore unwinding from inline assembly might not be sound or even UB.

[cynecx]

Ref: https://reviews.llvm.org/D95745

[cynecx]

UB when "non-unwindable" inline asm unwinds.

[jdoerfert]

OK, assuming we can make it UB that would help nounwind deduction. Nice.

[cynecx]

Ping.

[Amanieu]

LGTM

llvm/docs/LangRef.rst

 assumed dialect is ATT. When the '``inteldialect``' keyword is present,
 the inline asm is using the Intel dialect. Currently, ATT and Intel are
 the only supported dialects. An example is:
 
 .. code-block:: llvm
 
     call void asm inteldialect "eieio", ""()
 
-If multiple keywords appear the '``sideeffect``' keyword must come
-first, the '``alignstack``' keyword second and the '``inteldialect``'
-keyword last.
+In the case that the inline asm might unwind the stack,
+the '``unwind``' keyword must be used, so that the compiler emits
+unwinding information:
+
+.. code-block:: llvm
+
+    call void asm unwind "call func", ""()
+
+If the inline asm unwinds the stack and isn't marked with
+the '``unwind``' keyword, the behavior is undefined.
+
+If multiple keywords appear, the '``sideeffect``' keyword must come
+first, the '``alignstack``' keyword second, the '``inteldialect``' keyword
+third and the '``unwind``' keyword last.
 
 Inline Asm Constraint String
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 The constraint list is a comma-separated string, each element containing one or
 more constraint codes.
 
 For each element in the constraint list an appropriate register or memory