[glibc] Fix iconv build with GCC mainline

[Joseph Myers <jsm28@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=c8126360dfa98024cc40bce915e126309993cdf9

commit c8126360dfa98024cc40bce915e126309993cdf9
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Mon Aug 23 16:18:42 2021 +0000

    Fix iconv build with GCC mainline
    
    Current GCC mainline produces -Wstringop-overflow errors building some
    iconv converters, as discussed at
    <https://gcc.gnu.org/pipermail/gcc/2021-July/236943.html>.  Add an
    __builtin_unreachable call as suggested so that GCC can see the case
    that would involve a buffer overflow is unreachable; because the
    unreachability depends on valid conversion state being passed into the
    function from previous conversion steps, it's not something the
    compiler can reasonably deduce on its own.
    
    Tested with build-many-glibcs.py that, together with
    <https://sourceware.org/pipermail/libc-alpha/2021-August/130244.html>,
    it restores the glibc build for powerpc-linux-gnu.

Diff:
---
 iconv/loop.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/iconv/loop.c b/iconv/loop.c
index 062cc1b868..560a5f6394 100644
--- a/iconv/loop.c
+++ b/iconv/loop.c
@@ -436,6 +436,12 @@ SINGLE(LOOPFCT) (struct __gconv_step *step,
     return __GCONV_FULL_OUTPUT;
 
   /*  Now add characters from the normal input buffer.  */
+  if (inlen >= MAX_NEEDED_INPUT)
+    /* Avoid a -Wstringop-overflow= warning when this loop is
+       unrolled.  The compiler cannot otherwise see that this is
+       unreachable because it depends on (state->__count & 7) not
+       being too large after a previous conversion step.  */
+    __builtin_unreachable ();
   do
     bytebuf[inlen++] = *inptr++;
   while (inlen < MAX_NEEDED_INPUT && inptr < inend);