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

1 files changed, 3 insertions, 3 deletions

diff --git a/rts/sm/Scav.c b/rts/sm/Scav.c
index 1bee05221a..79adcaa826 100644
--- a/rts/sm/Scav.c
+++ b/rts/sm/Scav.c
@@ -337,7 +337,7 @@ scavenge_thunk_srt(const StgInfoTable *info)
     if (!major_gc) return;
 
     thunk_info = itbl_to_thunk_itbl(info);
-    if (thunk_info->i.has_srt) {
+    if (thunk_info->i.srt) {
         StgClosure *srt = (StgClosure*)GET_SRT(thunk_info);
         evacuate(&srt);
     }
@@ -351,7 +351,7 @@ scavenge_fun_srt(const StgInfoTable *info)
     if (!major_gc) return;
 
     fun_info = itbl_to_fun_itbl(info);
-    if (fun_info->i.has_srt) {
+    if (fun_info->i.srt) {
         StgClosure *srt = (StgClosure*)GET_FUN_SRT(fun_info);
         evacuate(&srt);
     }
@@ -1888,7 +1888,7 @@ scavenge_stack(StgPtr p, StgPtr stack_end)
         p = scavenge_small_bitmap(p, size, bitmap);
 
     follow_srt:
-        if (major_gc && info->i.has_srt) {
+        if (major_gc && info->i.srt) {
             StgClosure *srt = (StgClosure*)GET_SRT(info);
             evacuate(&srt);
         }