Re: Bootstrap broken for mipsel-linux: gcc/global.c:1020: error: array subscript is above array bounds

[Richard Sandiford <rsandifo at nildram dot co dot uk>]

David Daney <ddaney@avtrex.com> writes:
> For r131631 bootstrap on mipsel-linux in stage2 I am getting:
>
> /home/ddaney/gccsvn/trunk-build/./prev-gcc/xgcc 
> -B/home/ddaney/gccsvn/trunk-build/./prev-gcc/ 
> -B/home/ddaney/gccsvn/trunk-install/mipsel-linux/bin/ -c   -g -O2 
> -DIN_GCC   -W -Wall -Wwrite-strings -Wstrict-prototypes 
> -Wmissing-prototypes -Wold-style-definition -Wmissing-format-attribute 
> -pedantic -Wno-long-long 
> -Wno-variadic-macros                                 
> -Wno-overlength-strings -Werror -fno-common   -DHAVE_CONFIG_H -I. -I. 
> -I../../trunk/gcc -I../../trunk/gcc/. -I../../trunk/gcc/../include 
> -I../../trunk/gcc/../libcpp/include -I/home/ddaney/mp/include 
> -I/home/ddaney/mp/include -I../../trunk/gcc/../libdecnumber 
> -I../../trunk/gcc/../libdecnumber/dpd -I../libdecnumber    
> ../../trunk/gcc/final.c -o final.o
> /home/ddaney/gccsvn/trunk-build/./prev-gcc/xgcc 
> -B/home/ddaney/gccsvn/trunk-build/./prev-gcc/ 
> -B/home/ddaney/gccsvn/trunk-install/mipsel-linux/bin/ -c   -g -O2 
> -DIN_GCC   -W -Wall -Wwrite-strings -Wstrict-prototypes 
> -Wmissing-prototypes -Wold-style-definition -Wmissing-format-attribute 
> -pedantic -Wno-long-long 
> -Wno-variadic-macros                                 
> -Wno-overlength-strings -Werror -fno-common   -DHAVE_CONFIG_H -I. -I. 
> -I../../trunk/gcc -I../../trunk/gcc/. -I../../trunk/gcc/../include 
> -I../../trunk/gcc/../libcpp/include -I/home/ddaney/mp/include 
> -I/home/ddaney/mp/include -I../../trunk/gcc/../libdecnumber 
> -I../../trunk/gcc/../libdecnumber/dpd -I../libdecnumber    
> ../../trunk/gcc/global.c -o global.o
> cc1: warnings being treated as errors
> ../../trunk/gcc/global.c: In function 'find_reg':
> ../../trunk/gcc/global.c:1020: error: array subscript is above array bounds
> make[3]: *** [global.o] Error 1
>
> This is new since r131576:
>
> http://gcc.gnu.org/ml/gcc-testresults/2008-01/msg00758.html
>
> The configuration is identical to that successful bootstrap/test

global.c:1020 is new code that was added during that time.  There's
nothing intrinsically wrong with it though.  The warning is simply about
something that can't prove is dead code.  (I.e. it's another example in
favour of Mark's "the middle end shouldn't warn" thing.)  A reduced
testcase is:

----------------------------------------------------------------------
#define BIT(X, B) ((X) < ((B) ? 2 : 4) ? (X) + 4 : ~0U)

void bar (unsigned int *);
void foo (unsigned int b)
{
  unsigned int i, a[4];

  for (i = 0; BIT (i, b) != ~0U; i++)
    a[BIT (i, b) / 32] |= 1U << (BIT (i, b) % 32);
  bar (a);
}
----------------------------------------------------------------------

which generates a warning on x86_64-linux-gnu when compiled with -O2 -Wall.
We generate array accesses for a[(i + 4) / 32] and a[~0U / 32] and can't
prove that the latter are never used.

The attached patch uses the same construct as other E_R_D_R loops
and avoids the warning.  I'll test it overnight and install as obvious
if it succeeds.

Richard


Index: gcc/global.c
===================================================================
--- gcc/global.c	2008-01-19 23:54:32.000000000 +0000
+++ gcc/global.c	2008-01-19 23:54:56.000000000 +0000
@@ -1016,8 +1016,13 @@ find_reg (int num, HARD_REG_SET losers, 
   if (allocno[num].no_eh_reg)
     {
       unsigned int j;
-      for (j = 0; EH_RETURN_DATA_REGNO (j) != INVALID_REGNUM; j++)
-	SET_HARD_REG_BIT (used1, EH_RETURN_DATA_REGNO (j));
+      for (j = 0; ; ++j)
+	{
+	  unsigned int regno = EH_RETURN_DATA_REGNO (j);
+	  if (regno == INVALID_REGNUM)
+	    break;
+	  SET_HARD_REG_BIT (used1, regno);
+	}
     }
 #endif