Fix two bugs in stg_ap_0_fast in profiling runtime

This includes two bug fixes in profiling version of stg_ap_0_fast:

- PAPs allocated by stg_ap_0_fast are now correctly tagged. This
  invariant is checked in Sanity.c:checkPAP.

  (This was originally implemented in 2693eb11f5, later reverted with
  ab55b4ddb7 because it revealed the bug below, but it wasn't clear at
  the time whether the bug was the one below or something in the commit)

- The local variable `untaggedfun` is now marked as a pointer so it
  survives GC.

With this we finally fix all known bugs caught in #15508. `concprog001`
now works reliably with prof+threaded and prof runtimes (with and
without -debug).

(cherry picked from commit 908b4b8659713f0b7a1704ce33c7fa30e3e0ffc3)

1 files changed, 11 insertions, 7 deletions

diff --git a/rts/Apply.cmm b/rts/Apply.cmm
index 40f890d342..0454fd69e2 100644
--- a/rts/Apply.cmm
+++ b/rts/Apply.cmm
@@ -60,7 +60,7 @@ stg_ap_0_fast ( P_ fun )
 
 again:
     W_  info;
-    W_ untaggedfun;
+    P_ untaggedfun;
     W_ arity;
     untaggedfun = UNTAG(fun);
     info = %INFO_PTR(untaggedfun);
@@ -106,6 +106,11 @@ again:
                 pap = Hp - SIZEOF_StgPAP + WDS(1);
                 SET_HDR(pap, stg_PAP_info, CCCS);
                 StgPAP_arity(pap) = arity;
+                if (arity <= TAG_MASK) {
+                  // TODO: Shouldn't this already be tagged? If not why did we
+                  // untag it at the beginning of this function?
+                  fun = untaggedfun + arity;
+                }
                 StgPAP_fun(pap)   = fun;
                 StgPAP_n_args(pap) = 0;
                 return (pap);
@@ -117,9 +122,8 @@ again:
                 return (fun);
             } else {
                 // We're going to copy this PAP, and put the new CCS in it
-                fun = untaggedfun;
                 W_ size;
-                size = SIZEOF_StgPAP + WDS(TO_W_(StgPAP_n_args(fun)));
+                size = SIZEOF_StgPAP + WDS(TO_W_(StgPAP_n_args(untaggedfun)));
                 HP_CHK_GEN(size);
                 TICK_ALLOC_PAP(size, 0);
                 // attribute this allocation to the "overhead of profiling"
@@ -127,13 +131,13 @@ again:
                 P_ pap;
                 pap = Hp - size + WDS(1);
                 // We'll lose the original PAP, so we should enter its CCS
-                ccall enterFunCCS(BaseReg "ptr", StgHeader_ccs(fun) "ptr");
+                ccall enterFunCCS(BaseReg "ptr", StgHeader_ccs(untaggedfun) "ptr");
                 SET_HDR(pap, stg_PAP_info, CCCS);
-                StgPAP_arity(pap) = StgPAP_arity(fun);
-                StgPAP_n_args(pap) = StgPAP_n_args(fun);
+                StgPAP_arity(pap) = StgPAP_arity(untaggedfun);
+                StgPAP_n_args(pap) = StgPAP_n_args(untaggedfun);
                 StgPAP_fun(pap)   = StgPAP_fun(fun);
                 W_ i;
-                i = TO_W_(StgPAP_n_args(fun));
+                i = TO_W_(StgPAP_n_args(untaggedfun));
             loop:
                 if (i == 0) {
                     return (pap);