runtime: replace bubble sort with heap sort in select

R=golang-dev, agl
CC=golang-dev
https://golang.org/cl/7304106

diff --git a/src/pkg/runtime/chan.c b/src/pkg/runtime/chan.c
index 9b915cef013f1df00e476ae1fe362217c85e6a96..a15b5d0d1af56743b393b2fff01f3002c3d58e85 100644 (file)
--- a/src/pkg/runtime/chan.c
+++ b/src/pkg/runtime/chan.c
@@ -840,7 +840,7 @@ static void*
 selectgo(Select **selp)
 {
        Select *sel;
-       uint32 o, i, j;
+       uint32 o, i, j, k;
        Scase *cas, *dfl;
        Hchan *c;
        SudoG *sg;
@@ -874,12 +874,42 @@ selectgo(Select **selp)
        }
 
        // sort the cases by Hchan address to get the locking order.
+       // simple heap sort, to guarantee n log n time and constant stack footprint.
        for(i=0; i<sel->ncase; i++) {
-               c = sel->scase[i].chan;
-               for(j=i; j>0 && sel->lockorder[j-1] >= c; j--)
-                       sel->lockorder[j] = sel->lockorder[j-1];
+               j = i;
+               c = sel->scase[j].chan;
+               while(j > 0 && sel->lockorder[k=(j-1)/2] < c) {
+                       sel->lockorder[j] = sel->lockorder[k];
+                       j = k;
+               }
+               sel->lockorder[j] = c;
+       }
+       for(i=sel->ncase; i-->0; ) {
+               c = sel->lockorder[i];
+               sel->lockorder[i] = sel->lockorder[0];
+               j = 0;
+               for(;;) {
+                       k = j*2+1;
+                       if(k >= i)
+                               break;
+                       if(k+1 < i && sel->lockorder[k] < sel->lockorder[k+1])
+                               k++;
+                       if(c < sel->lockorder[k]) {
+                               sel->lockorder[j] = sel->lockorder[k];
+                               j = k;
+                               continue;
+                       }
+                       break;
+               }
                sel->lockorder[j] = c;
        }
+       /*
+       for(i=0; i+1<sel->ncase; i++)
+               if(sel->lockorder[i] > sel->lockorder[i+1]) {
+                       runtime·printf("i=%d %p %p\n", i, sel->lockorder[i], sel->lockorder[i+1]);
+                       runtime·throw("select: broken sort");
+               }
+       */
        sellock(sel);
 
 loop: