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Turn eval into evali

This is based on the observation that two things can happen in eval:
* The stack is clear, in which case the value in n is a direct
  reference to the WHNF of the original argument
* The stack is not clear, in which case the top of stack is an AP
  representing the argument without indirections.

In either case there's no need to push / pop it again and go chasing
after indirections.

This represents a small time savings of about .2s on bootstrap, see
https://docs.google.com/spreadsheets/d/1Tw6LLwTEU0VxdWcf_ZUF1_T9KlchpNEFXKNiAarxd4I
which shows data from 10 interleaved after/before runs plus some
statistically bootstrapped averages with histograms showing limited overlap.

--- a/src/runtime/eval.c

+++ b/src/runtime/eval.c

@@ -1897,21 +1897,8 @@

   return n;

 }

-void eval(NODEPTR n);

+NODEPTR evali(NODEPTR n);

-/* Evaluate and skip indirections. */

-static INLINE NODEPTR

-evali(NODEPTR n)

-{

-  /* Need to push and pop in case GC happens */

-  PUSH(n);

-  eval(n);

-  n = POPTOP();

-  while (GETTAG(n) == T_IND)

-    n = INDIR(n);

-  return n;

-}

-

 /* Follow indirections */

 static INLINE NODEPTR

 indir(NODEPTR n)

@@ -2141,8 +2128,8 @@

 void execio(NODEPTR *);

 /* Evaluate a node, returns when the node is in WHNF. */

-void

-eval(NODEPTR an)

+NODEPTR

+evali(NODEPTR an)

 {

   NODEPTR n = an;

   stackptr_t stk = stack_ptr;

@@ -2158,7 +2145,7 @@

 #endif

 /* Reset stack pointer and return. */

-#define RET do { stack_ptr = stk; return; } while(0)

+#define RET do { goto ret; } while(0)

 /* Check that there are at least n arguments, return if not. */

 #define CHECK(n) do { if (stack_ptr - stk < (n)) RET; } while(0)

@@ -2402,7 +2389,7 @@

         ERR1("error: %s", msg);

 #endif  /* WANT_STDIO */

       }

-    case T_SEQ:  CHECK(2); eval(ARG(TOP(0))); POP(2); n = TOP(-1); y = ARG(n); GOIND(y); /* seq x y = eval(x); y */

+    case T_SEQ:  CHECK(2); evali(ARG(TOP(0))); POP(2); n = TOP(-1); y = ARG(n); GOIND(y); /* seq x y = eval(x); y */

     case T_EQUAL:

       CHECK(2); r = compare(TOP(1)); POP(2); n = TOP(-1); GOIND(r==0 ? combTrue : combFalse);

@@ -2469,6 +2456,14 @@

       ERR1("eval tag %d", GETTAG(n));

     }

   }

+ ret:

+  if (stack_ptr != stk) {

+    // In this case, n was an AP that got pushed and potentially

+    // updated.

+    stack_ptr = stk;

+    n = TOP(-1);

+  }

+  return n;

 }

 /* This is the interpreter for the IO monad operations.

-- 

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