PEI: refactor replaceFrameIndices(MF) to call replaceFrameIndices(BB).

[oota-llvm.git] / docs / FAQ.rst

diff --git a/docs/FAQ.rst b/docs/FAQ.rst
index 67e8d0b27b26ee4ba991b638b6bd24c6643eb639..e4ab2c18f748073ae79d060767e51c7652875677 100644 (file)
--- a/docs/FAQ.rst
+++ b/docs/FAQ.rst
@@ -1,5 +1,3 @@
-.. _faq:
-

 ================================
 Frequently Asked Questions (FAQ)
 ================================
 ================================
 Frequently Asked Questions (FAQ)
 ================================


@@ -53,6 +51,29 @@ Some porting problems may exist in the following areas:
   like the Bourne Shell and sed.  Porting to systems without these tools
   (MacOS 9, Plan 9) will require more effort.
 
   like the Bourne Shell and sed.  Porting to systems without these tools
   (MacOS 9, Plan 9) will require more effort.
 

+What API do I use to store a value to one of the virtual registers in LLVM IR's SSA representation?
+---------------------------------------------------------------------------------------------------
+
+In short: you can't. It's actually kind of a silly question once you grok
+what's going on. Basically, in code like:
+
+.. code-block:: llvm
+
+    %result = add i32 %foo, %bar
+
+, ``%result`` is just a name given to the ``Value`` of the ``add``
+instruction. In other words, ``%result`` *is* the add instruction. The
+"assignment" doesn't explicitly "store" anything to any "virtual register";
+the "``=``" is more like the mathematical sense of equality.
+
+Longer explanation: In order to generate a textual representation of the
+IR, some kind of name has to be given to each instruction so that other
+instructions can textually reference it. However, the isomorphic in-memory
+representation that you manipulate from C++ has no such restriction since
+instructions can simply keep pointers to any other ``Value``'s that they
+reference. In fact, the names of dummy numbered temporaries like ``%1`` are
+not explicitly represented in the in-memory representation at all (see
+``Value::getName()``).

 
 Build Problems
 ==============
 
 Build Problems
 ==============