Save a word in the info table on x86_64

Summary:
An info table with an SRT normally looks like this:

    StgWord64 srt_offset
    StgClosureInfo layout
    StgWord32 layout
    StgWord32 has_srt

But we only need 32 bits for srt_offset on x86_64, because the small
memory model requires that code segments are at most 2GB. So we can
optimise this to

    StgClosureInfo layout
    StgWord32 layout
    StgWord32 srt_offset

saving a word.  We can tell whether the info table has an SRT or not,
because zero is not a valid srt_offset, so zero still indicates that
there's no SRT.

Test Plan:
* validate
* For results, see D4632.

Reviewers: bgamari, niteria, osa1, erikd

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D4634

2 files changed, 40 insertions, 10 deletions

diff --git a/includes/rts/storage/ClosureMacros.h b/includes/rts/storage/ClosureMacros.h
index 2b78ab44b8..7a3ecaa1d2 100644
--- a/includes/rts/storage/ClosureMacros.h
+++ b/includes/rts/storage/ClosureMacros.h
@@ -109,7 +109,7 @@ INLINE_HEADER const StgConInfoTable *get_con_itbl(const StgClosure *c)
 
 INLINE_HEADER StgHalfWord GET_TAG(const StgClosure *con)
 {
-    return get_itbl(con)->has_srt;
+    return get_itbl(con)->srt;
 }
 
 /* -----------------------------------------------------------------------------
diff --git a/includes/rts/storage/InfoTables.h b/includes/rts/storage/InfoTables.h
index 0e25e14c8e..8107510321 100644
--- a/includes/rts/storage/InfoTables.h
+++ b/includes/rts/storage/InfoTables.h
@@ -153,6 +153,21 @@ typedef union {
 } StgClosureInfo;
 
 
+#if defined(x86_64_TARGET_ARCH) && defined(TABLES_NEXT_TO_CODE)
+// On x86_64 we can fit a pointer offset in half a word, so put the SRT offset
+// in the info->srt field directly.
+#define USE_INLINE_SRT_FIELD
+#endif
+
+#if defined(USE_INLINE_SRT_FIELD)
+// offset to the SRT / closure, or zero if there's no SRT
+typedef StgHalfInt StgSRTField;
+#else
+// non-zero if there is an SRT, the offset is in the optional srt field.
+typedef StgHalfWord StgSRTField;
+#endif
+
+
 /*
  * The "standard" part of an info table.  Every info table has this bit.
  */
@@ -169,11 +184,14 @@ typedef struct StgInfoTable_ {
     StgClosureInfo  layout;     /* closure layout info (one word) */
 
     StgHalfWord     type;       /* closure type */
-    StgHalfWord     has_srt;
+    StgSRTField     srt;
        /* In a CONSTR:
             - the constructor tag
           In a FUN/THUNK
-            - non-zero if there is an SRT
+            - if USE_INLINE_SRT_FIELD
+              - offset to the SRT (or zero if no SRT)
+            - otherwise
+              - non-zero if there is an SRT, offset is in srt_offset
        */
 
 #if defined(TABLES_NEXT_TO_CODE)
@@ -214,7 +232,9 @@ typedef struct StgFunInfoExtraRev_ {
         StgWord bitmap;
         OFFSET_FIELD(bitmap_offset);    /* arg ptr/nonptr bitmap */
     } b;
+#if !defined(USE_INLINE_SRT_FIELD)
     OFFSET_FIELD(srt_offset);   /* pointer to the SRT closure */
+#endif
     StgHalfWord    fun_type;    /* function type */
     StgHalfWord    arity;       /* function arity */
 } StgFunInfoExtraRev;
@@ -253,7 +273,9 @@ extern const StgWord stg_arg_bitmaps[];
 
 typedef struct {
 #if defined(TABLES_NEXT_TO_CODE)
+#if !defined(USE_INLINE_SRT_FIELD)
     OFFSET_FIELD(srt_offset);   /* offset to the SRT closure */
+#endif
     StgInfoTable i;
 #else
     StgInfoTable i;
@@ -271,16 +293,14 @@ typedef struct {
  */
 
 typedef struct StgThunkInfoTable_ {
-#if !defined(TABLES_NEXT_TO_CODE)
-    StgInfoTable i;
-#endif
 #if defined(TABLES_NEXT_TO_CODE)
+#if !defined(USE_INLINE_SRT_FIELD)
     OFFSET_FIELD(srt_offset);   /* offset to the SRT closure */
-#else
-    StgClosure  *srt;           /* pointer to the SRT closure */
 #endif
-#if defined(TABLES_NEXT_TO_CODE)
     StgInfoTable i;
+#else
+    StgInfoTable i;
+    StgClosure  *srt;           /* pointer to the SRT closure */
 #endif
 } StgThunkInfoTable;
 
@@ -315,9 +335,14 @@ typedef struct StgConInfoTable_ {
  * info must be a Stg[Ret|Thunk]InfoTable* (an info table that has a SRT)
  */
 #if defined(TABLES_NEXT_TO_CODE)
+#if x86_64_TARGET_ARCH
 #define GET_SRT(info) \
-  ((StgClosure*) (((StgWord) ((info)+1)) + (info)->srt_offset))
+  ((StgClosure*) (((StgWord) ((info)+1)) + (info)->i.srt))
 #else
+#define GET_SRT(info) \
+  ((StgClosure*) (((StgWord) ((info)+1)) + (info)->srt_offset))
+#endif
+#else // !TABLES_NEXT_TO_CODE
 #define GET_SRT(info) ((info)->srt)
 #endif
 
@@ -337,8 +362,13 @@ typedef struct StgConInfoTable_ {
  * info must be a StgFunInfoTable*
  */
 #if defined(TABLES_NEXT_TO_CODE)
+#if x86_64_TARGET_ARCH
+#define GET_FUN_SRT(info) \
+  ((StgClosure*) (((StgWord) ((info)+1)) + (info)->i.srt))
+#else
 #define GET_FUN_SRT(info) \
   ((StgClosure*) (((StgWord) ((info)+1)) + (info)->f.srt_offset))
+#endif
 #else
 #define GET_FUN_SRT(info) ((info)->f.srt)
 #endif