[Git][ghc/ghc][master] rts: drop stale mentions of MIN_UPD_SIZE

[Marge Bot (@marge-bot)]

Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
c1e3719c by Cheng Shao at 2023-10-22T02:49:33-04:00
rts: drop stale mentions of MIN_UPD_SIZE

We used to have MIN_UPD_SIZE macro that describes the minimum reserved
size for thunks, so that the thunk can be overwritten in place as
indirections or blackholes. However, this macro has not been actually
defined or used anywhere since a long time ago; StgThunkHeader already
reserves a padding word for this purpose. Hence this patch which drops
stale mentions of MIN_UPD_SIZE.

- - - - -


2 changed files:

- docs/rts/rts.tex
- rts/PrimOps.cmm


Changes:

=====================================
docs/rts/rts.tex
=====================================
@@ -1970,10 +1970,9 @@ Here the right-hand sides of @range@ and @ys@ are both thunks; the former
 is static while the latter is dynamic.
 
 The layout of a thunk is the same as that for a function closure.
-However, thunks must have a payload of at least @MIN_UPD_SIZE@
-words to allow it to be overwritten with a black hole and an
-indirection.  The compiler may have to add extra non-pointer fields to
-satisfy this constraint.
+However, a thunk header always contains an extra padding word at the
+end. This allows the thunk to be overwritten with an indirection,
+where the padding word will be repurposed as the indirectee pointer.
 
 \begin{center}
 \begin{tabular}{|l|l|l|l|l|}\hline


=====================================
rts/PrimOps.cmm
=====================================
@@ -740,25 +740,15 @@ stg_atomicModifyMutVar2zh ( gcptr mv, gcptr f )
 
         obviously we can share (f x).
 
-         z = [stg_ap_2 f x]  (max (HS + 2) MIN_UPD_SIZE)
-         y = [stg_sel_0 z]   (max (HS + 1) MIN_UPD_SIZE)
+         z = [stg_ap_2 f x]  (SIZEOF_StgThunkHeader + WDS(2))
+         y = [stg_sel_0 z]   (SIZEOF_StgThunkHeader + WDS(1))
     */
 
-#if defined(MIN_UPD_SIZE) && MIN_UPD_SIZE > 1
-#define THUNK_1_SIZE (SIZEOF_StgThunkHeader + WDS(MIN_UPD_SIZE))
-#define TICK_ALLOC_THUNK_1() TICK_ALLOC_UP_THK(WDS(1),WDS(MIN_UPD_SIZE-1))
-#else
 #define THUNK_1_SIZE (SIZEOF_StgThunkHeader + WDS(1))
 #define TICK_ALLOC_THUNK_1() TICK_ALLOC_UP_THK(WDS(1),0)
-#endif
 
-#if defined(MIN_UPD_SIZE) && MIN_UPD_SIZE > 2
-#define THUNK_2_SIZE (SIZEOF_StgThunkHeader + WDS(MIN_UPD_SIZE))
-#define TICK_ALLOC_THUNK_2() TICK_ALLOC_UP_THK(WDS(2),WDS(MIN_UPD_SIZE-2))
-#else
 #define THUNK_2_SIZE (SIZEOF_StgThunkHeader + WDS(2))
 #define TICK_ALLOC_THUNK_2() TICK_ALLOC_UP_THK(WDS(2),0)
-#endif
 
 #define SIZE (THUNK_2_SIZE + THUNK_1_SIZE)
 
@@ -815,13 +805,8 @@ stg_atomicModifyMutVarzuzh ( gcptr mv, gcptr f )
          z = [stg_ap_2 f x]  (max (HS + 2) MIN_UPD_SIZE)
     */
 
-#if defined(MIN_UPD_SIZE) && MIN_UPD_SIZE > 2
-#define THUNK_SIZE (SIZEOF_StgThunkHeader + WDS(MIN_UPD_SIZE))
-#define TICK_ALLOC_THUNK() TICK_ALLOC_UP_THK(WDS(2),WDS(MIN_UPD_SIZE-2))
-#else
 #define THUNK_SIZE (SIZEOF_StgThunkHeader + WDS(2))
 #define TICK_ALLOC_THUNK() TICK_ALLOC_UP_THK(WDS(2),0)
-#endif
 
     HP_CHK_GEN_TICKY(THUNK_SIZE);
 



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/c1e3719cc59f20d32c917e066ae7cd5edb98a96d

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/c1e3719cc59f20d32c917e066ae7cd5edb98a96d
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