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Move trivial colorability checks into a function.

    This will make it easier to check for colorability in the face of
    overlapping register classes.

--- a/6/asm.h

+++ b/6/asm.h

@@ -3,7 +3,7 @@

 #define Maxdef (2*Maxarg)       /* maximum number of registers an insn can use or def */

 #define Wordsz 4                /* the size of a "natural int" */

 #define Ptrsz 8                 /* the size of a machine word (ie, pointer size) */

-#define K 14                    /* the number of allocatable registers */

+#define _K 14                    /* the number of allocatable registers */

 #define Nsaved 13               /* number of registers saved in the ABI */

 typedef size_t regid;

--- a/6/ra.c

+++ b/6/ra.c

@@ -199,6 +199,11 @@

     }

 }

+static int istrivial(Isel *s, regid r)

+{

+    return s->degree[r] < _K;

+}

+

 static void liveness(Isel *s)

 {

     Bitset *old;

@@ -485,7 +490,7 @@

     for (i = 0; bsiter(s->initial, &i); i++) {

         if (bshas(s->prepainted, i))

             continue;

-        else if (s->degree[i] >= K)

+        else if (!istrivial(s, i))

             lappend(&s->wlspill, &s->nwlspill, locmap[i]);

         else if (moverelated(s, i))

             lappend(&s->wlfreeze, &s->nwlfreeze, locmap[i]);

@@ -514,16 +519,17 @@

 static void decdegree(Isel *s, regid m)

 {

-    int d;

+    int before, after;

     int found;

     size_t idx;

     regid n;

     assert(m < maxregid);

-    d = s->degree[m];

+    before = istrivial(s, m);

     s->degree[m]--;

+    after = istrivial(s, m);

-    if (d == K) {

+    if (before != after) {

         enablemove(s, m);

         for (n = 0; adjiter(s, m, &n); n++)

             enablemove(s, n);

@@ -586,7 +592,7 @@

         return;

     if (moverelated(s, u))

         return;

-    if (s->degree[u] >= K)

+    if (!istrivial(s, u))

         return;

     assert(wlhas(s->wlfreeze, s->nwlfreeze, u, &i));

@@ -601,18 +607,18 @@

     k = 0;

     for (n = 0; adjiter(s, u, &n); n++)

-        if (s->degree[n] >= K)

+        if (!istrivial(s, n))

             k++;

     for (n = 0; adjiter(s, v, &n); n++)

-        if (s->degree[n] >= K)

+        if (!istrivial(s, n))

             k++;

-    return k < K;

+    return k < _K;

 }

 /* FIXME: is this actually correct? */

 static int ok(Isel *s, regid t, regid r)

 {

-    return s->degree[t] < K || bshas(s->prepainted, t) || gbhasedge(s, t, r);

+    return istrivial(s, t) || bshas(s->prepainted, t) || gbhasedge(s, t, r);

 }

 static int combinable(Isel *s, regid u, regid v)

@@ -671,7 +677,7 @@

         addedge(s, t, u);

         decdegree(s, t);

     }

-    if (s->degree[u] >= K && wlhas(s->wlfreeze, s->nwlfreeze, u, &idx)) {

+    if (!istrivial(s, u) && wlhas(s->wlfreeze, s->nwlfreeze, u, &idx)) {

         ldel(&s->wlfreeze, &s->nwlfreeze, idx);

         lappend(&s->wlspill, &s->nwlspill, locmap[u]);

     }

@@ -741,7 +747,7 @@

         if (!mldel(&s->mactive, &s->nmactive, m))

             mldel(&s->wlmove, &s->nwlmove, m);

         lappend(&s->mfrozen, &s->nmfrozen, m);

-        if (!nodemoves(s, v->reg.id, NULL) && s->degree[v->reg.id] < K) {

+        if (!nodemoves(s, v->reg.id, NULL) && istrivial(s, v->reg.id)) {

             if (!wlhas(s->wlfreeze, s->nwlfreeze, v->reg.id, &idx))

                 die("Reg %zd not in freeze wl\n", v->reg.id);

             ldel(&s->wlfreeze, &s->nwlfreeze, idx);

@@ -798,7 +804,7 @@

  */

 static int paint(Isel *s)

 {

-    int taken[K + 2]; /* esp, ebp aren't "real colours" */

+    int taken[_K + 2]; /* esp, ebp aren't "real colours" */

     Loc *n, *w;

     regid l;

     size_t i;

@@ -807,7 +813,7 @@

     spilled = 0;

     while (s->nselstk) {

-        bzero(taken, K*sizeof(int));

+        bzero(taken, _K*sizeof(int));

         n = lpop(&s->selstk, &s->nselstk);

         for (l = 0; bsiter(s->gadj[n->reg.id], &l); l++) {

@@ -819,7 +825,7 @@

         }

         found = 0;

-        for (i = 0; i < K; i++) {

+        for (i = 0; i < _K; i++) {

             if (!taken[i]) {

                 if (debugopt['r']) {

                     fprintf(stdout, "\tselecting ");