Invalid loop extent reduction

[miklos1]

COFFEE did the following transformation:

--- /tmp/pyop2-cache-uid11480/95256e7f5216f51a1d88d64fe2f433bc_p5040.c  2016-10-20 18:15:50.711984816 +0100
+++ /tmp/pyop2-cache-uid11480/16b9d6342d4efc75d2e86893dd6db688_p5237.c  2016-10-20 18:15:25.199611203 +0100
@@ -38,13 +38,23 @@
   for (int  ip_1  = 0; ip_1 < 2; ip_1 += 1)
   {

+    for (int  k0  = 0; k0 < 2; k0 += 1)
+    {
+      
+      for (int  k1  = 0; k1 < 2; k1 += 1)
+      {
+        t8[ip_1][k0][k1] = t2[k0] * t6[ip_1][k1];
+        
+      }
+      
+    }
+    
     for (int  k0  = 0; k0 < 3; k0 += 1)
     {

       for (int  k1  = 0; k1 < 2; k1 += 1)
       {
         t7[ip_1][k0][k1] = t1[k0] * t6[ip_1][k1];
-        t8[ip_1][k0][k1] = t2[k0] * t6[ip_1][k1];

       }  

This reduces the extent of the k0 loop from 3 to 2 for t8[ip_1][k0][k1] = t2[k0] * t6[ip_1][k1];. This would be legal, had t8 been previously initialised to zero, but it wasn't. The loop that was reduced was supposed to initialise it, thus the reduced loop leaving entries uninitialised, and causing all kind of horrible symptoms.

[miklos1]

@FabioLuporini: Just wanted to document this, so we won't forget this problem. Not urgent to fix it, since it is not triggered per firedrakeproject/tsfc@1d7ce21.

[FabioLuporini]

OK, thanks.

[FabioLuporini]

I prefer to keep this open for reference