Re: [PATCH] Avoid invoking undefined behavior when initializing CRC table

[Patrick Palka <patrick at parcs dot ath dot cx>]

On Sun, 27 Dec 2015, Patrick Palka wrote:

> When I built GDB with (an older snapshot of) GCC 6 I get the following
> error:
>
> .../binutils-gdb/gdb/gdbserver/server.c: In function âcrc32â:
> .../binutils-gdb/gdb/gdbserver/server.c:1895:15: error: iteration 128 invokes undefined behavior [-Werror=aggressive-loop-optimizations]
>    for (c = i << 24, j = 8; j > 0; --j)
>               ^
> .../binutils-gdb/gdb/gdbserver/server.c:1893:7: note: within this loop
>       for (i = 0; i < 256; i++)
>       ^
> This error seems to be correct.  When the variable "int i" is >= 128,
> the computation "i << 24" overflows for 32-bit signed int.
>
> To avoid shifting into the sign bit, this patch makes the variables i
> (and j, because why not) have type unsigned int instead.
>
> (Alternatively, I can just define this local crc32 function in terms of
> libiberty's xcrc32.  Any reason not to?  xcrc32 seems to be
> based off of GDB's crc32 implementation.  Its documentation even
> refers to it!)

And here's a rough diff that defines crc32 in terms of xcrc32:

diff --git a/gdb/gdbserver/server.c b/gdb/gdbserver/server.c
index 70acafc..0e3ac4e 100644
--- a/gdb/gdbserver/server.c
+++ b/gdb/gdbserver/server.c
@@ -1911,11 +1911,6 @@ handle_qxfer (char *own_buf, int packet_len, int *new_packet_len_p)
   return 0;
 }

-/* Table used by the crc32 function to calcuate the checksum.  */
-
-static unsigned int crc32_table[256] =
-{0, 0};
-
 /* Compute 32 bit CRC from inferior memory.

    On success, return 32 bit CRC.
@@ -1924,20 +1919,6 @@ static unsigned int crc32_table[256] =
 static unsigned long long
 crc32 (CORE_ADDR base, int len, unsigned int crc)
 {
-  if (!crc32_table[1])
-    {
-      /* Initialize the CRC table and the decoding table.  */
-      unsigned int i, j;
-      unsigned int c;
-
-      for (i = 0; i < 256; i++)
-       {
-         for (c = i << 24, j = 8; j > 0; --j)
-           c = c & 0x80000000 ? (c << 1) ^ 0x04c11db7 : (c << 1);
-         crc32_table[i] = c;
-       }
-    }
-
   while (len--)
     {
       unsigned char byte = 0;
@@ -1946,7 +1927,7 @@ crc32 (CORE_ADDR base, int len, unsigned int crc)
       if (read_inferior_memory (base, &byte, 1) != 0)
        return (unsigned long long) -1;

-      crc = (crc << 8) ^ crc32_table[((crc >> 24) ^ byte) & 255];
+      crc = xcrc32 (&byte, 1, crc);
       base++;
     }
   return (unsigned long long) crc;