code review 76730046: cmd/gc: check exponent overflow and underflow in mparith

src/cmd/gc/mparith1.c

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 #include        <u.h>
 #include        <libc.h>
 #include        "go.h"
 
 /// uses arithmetic
 
@@ skipping 398 matching lines @@
 
                 case 0:
                         break;
                 }
                 break;
         }
 
         if(eb) {
                 if(dp)
                         goto bad;
-                a->exp += ex;
+                mpsetexp(a, a->exp+ex);
                 goto out;
         }
 
         if(dp)
                 dp--;
         if(mpcmpfltc(a, 0.0) != 0) {
                 if(ex >= dp) {
                         mppow10flt(&b, ex-dp);
                         mpmulfltflt(a, &b);
                 } else {
-                        mppow10flt(&b, dp-ex);
-                        mpdivfltflt(a, &b);
+                        if((short)(dp-ex) != dp-ex) {

[iant, 2014/03/18 21:17:40]

I don't quite see why this check is necessary.  It seems to me that if it is not
true, it will still be caught by the check in mpdivfltflt.

[atom, 2014/03/18 21:28:59]

We want 1/VeryLargeNumber to be rounded to zero, so we want to avoid the error.

+                                mpmovecflt(a, 0.0);
+                        }
+                        else {
+                                mppow10flt(&b, dp-ex);
+                                mpdivfltflt(a, &b);
+                        }
                 }
         }
 
 out:
         if(f)
                 mpnegflt(a);
         return;
 
 bad:
         yyerror("constant too large: %s", as);
@@ skipping 164 matching lines @@
 
         if(fv.exp >= 0) {
                 snprint(buf, sizeof(buf), "%#Bp+%d", &fv.val, fv.exp);
                 goto out;
         }
         snprint(buf, sizeof(buf), "%#Bp-%d", &fv.val, -fv.exp);
 
 out:
         return fmtstrcpy(fp, buf);
 }