Changes between Version 2 and Version 3 of Commentary/Compiler/Backends/PprC

Timestamp:

04/09/08 13:32:18 (5 years ago)

Author:

simonmar

Comment:

add information about prototypes

Commentary/Compiler/Backends/PprC

@@ -21 +21 @@
 This has several advantages:
   
-  * `-fvia-C` is consistent with `-fasm` with respect to FFI declarations:
-     both bind to the ABI, not the API.
+ * `-fvia-C` is consistent with `-fasm` with respect to FFI declarations:
+   both bind to the ABI, not the API.
   
-  * foreign calls can now be inlined freely across module boundaries, since
-    a header file is not required when compiling the call.
+ * foreign calls can now be inlined freely across module boundaries, since
+   a header file is not required when compiling the call.
   
-  * bootstrapping via C will be more reliable, because this difference
-    in behavour between the two backends has been removed.
+ * bootstrapping via C will be more reliable, because this difference
+   in behavour between the two backends has been removed.
   
-  There is one disadvantage:
+There is one disadvantage:
   
-  * we get no checking by the C compiler that the FFI declaration
-    is correct.
+ * we get no checking by the C compiler that the FFI declaration
+   is correct.
   
 == Prototypes ==
 
+When a label is referenced by an expression, the compiler needs to
+know whether to declare the label first, and if so, at what type.
+
+ * all labels referenced as a result of an FFI declaration
+   are declared as `extern StgWord[]`, including funciton labels.
+   If the label is called, it is first cast to the correct
+   function type.  This is because the same label might be
+   referred to both as a function and an untyped data label in
+   the same module (e.g. Foreign.Marsal.Alloc refers to "free"
+   this way).
+
+ * all RTS symbols already have declarations (mostly with the correct
+   type) in [[GhcFile(includes/StgMiscClosures.h)]], so no declarations are generated.
+
+ * certain labels are known to have been defined earlier in the same file,
+   so a declaration can be omitted (e.g. SRT labels)
+
+ * certain math functions (`sin()`, `cos()` etc.) are already declared because
+   we #include math.h, so we don't emit declarations for these.  We need
+   to #include math.h because some of these fuctions have inline
+   definitions, and we get terrible code otherwise.
+
+When compiling the RTS cmm code, we have almost no information about
+labels referenced in the code.  The only information we have is
+whether the label is defined in the RTS or in another package: a label
+that is declared with an import statement in the .cmm file is assumed
+to be defined in another package (this is for dynamic linking, where
+we need to emit special code to reference these labels).
+
+For all other labels referenced by RTS .cmm code, we assume they are
+RTS labels, and hence already declared in [[GhcFile(includes/StgMiscClosures.h)]].  This is
+the only choice here: since we don't know the type of the label (info,
+entry etc.), we can't generate a correct declaration.
+