Small optimization in the SRT algorithm

Noticed by @simonmar in !1362:

    If the srtEntry is Nothing, then it should be safe to omit
    references to this SRT from other SRTs, even if it is a static
    function.

When updating SRT map we don't omit references to static functions (see
Note [Invalid optimisation: shortcutting]), but there's no reason to add
an SRT entry for a static function if the function is not CAFFY.

(Previously we'd add SRT entries for static functions even when they're
not CAFFY)

Using 9151b99e I checked sizes of all SRTs when building GHC and
containers:

- GHC: 583736 (HEAD), 581695 (this patch). 2041 less SRT entries.
- containers: 2457 (HEAD), 2381 (this patch). 76 less SRT entries.

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/cmm/CmmBuildInfoTables.hs b/compiler/cmm/CmmBuildInfoTables.hs
index 9c05a8fe29..bde52de3af 100644
--- a/compiler/cmm/CmmBuildInfoTables.hs
+++ b/compiler/cmm/CmmBuildInfoTables.hs
@@ -727,7 +727,7 @@ oneSRT dflags staticFuns blockids lbls isCAF cafs = do
     -- important that we don't do this for static functions or CAFs,
     -- see Note [Invalid optimisation: shortcutting].
     updateSRTMap srtEntry =
-      when (not isCAF && not isStaticFun) $ do
+      when (not isCAF && (not isStaticFun || isNothing srtEntry)) $ do
         let newSRTMap = Map.fromList [(cafLbl, srtEntry) | cafLbl <- lbls]
         put (Map.union newSRTMap srtMap)