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* [Bug c/102844] New: gcc-9.x miscompiles unsigned char expression in switch
@ 2021-10-19 17:45 breiten at lexmark dot com
  2021-10-19 17:58 ` [Bug tree-optimization/102844] [9/10/11/12 Regression] " pinskia at gcc dot gnu.org
                   ` (33 more replies)
  0 siblings, 34 replies; 35+ messages in thread
From: breiten at lexmark dot com @ 2021-10-19 17:45 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102844

            Bug ID: 102844
           Summary: gcc-9.x miscompiles unsigned char expression in switch
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: breiten at lexmark dot com
  Target Milestone: ---

The code included below demonstrates a code-generation bug we found in gcc-9.3
(built with poky's dunfell branch) It is present in all of the 9.x releases,
but none of the 10.x releases.

Anticipating your first question, I found that this problem is NOT present
in gcc-10.2, so I went earlier in that branch and was able to see that it
was corrected by 744fd446c321f78f9a1ce4ef5f83df8dcfa44a9 (gcc-10 branch,
but before gcc-10.1 release) Nothing in the commit messages suggests a
bug fix.

I've also been able to pinpoint the commit where the problem first appears
in the gcc-9 branch: 1a438d160e1dc845882c2ecba99513a09a931623.  Here, the
commit message describes this commit to be a bug fix.

My dilemma is that I'm stuck in the 9 branch for a while.  Reverting a bug
fix doesn't feel right. But while picking up the commit from gcc-10 does
appear to work, it a bit more than a simple bug-fix, and I'd like to hear
some opinions regarding its appropriateness.

Maybe there it still a bug out there, and the commit from gcc-10 just makes
this particular code compile properly.

In the C source below, line 58 miscompiles into an unconditional store of
0x80.  Notice that I have two switch statements on the same variable and
with the same cases.  If I remove or alter the other one, the bad code and
the warning goes away.  It also is only present when "nd - 0x80" is stored
to a uchar variable.  If its stored to an int or printf'd the code is correct.
Also, adding "U" suffix to 0x80 corrects the expression.  And I get the warning
regardless of what number I subtract from nd, suggesting that it is incorrectly
considered a signed char, and subtracting anything would underflow.

I see the problem with -Os or -O2, and -fno-strict-overflow is needed.  In
cleaning up the command line, I found that linux adds -Wno-maybe-uninitialized.
Removing that leads to this message:

attach3.c:60:22: warning: '({anonymous})' may be used uninitialized in this
function [-Wmaybe-uninitialized]
   58 |             *ic = nd - 0x80; /* compiles into an unconditional store of
0x80 */
      |                   ~~~^~~~~~

Here's my gcc configuration:
Target: arm-poky-linux-gnueabi
Configured with: ../../../../../../work-shared/gcc-9.3.0-r0/gcc-9.3.0/configure
--build=x86_64-linux --host=x86_64-linux --target=arm-poky-linux-gnueabi
--prefix=/host-native/usr --exec_prefix=/host-native/usr
--bindir=/host-native/usr/bin/arm-poky-linux-gnueabi
--sbindir=/host-native/usr/bin/arm-poky-linux-gnueabi
--libexecdir=/host-native/usr/libexec/arm-poky-linux-gnueabi
--datadir=/host-native/usr/share --sysconfdir=/host-native/etc
--sharedstatedir=/host-native/com --localstatedir=/host-native/var
--libdir=/host-native/usr/lib/arm-poky-linux-gnueabi
--includedir=/host-native/usr/include --oldincludedir=/host-native/usr/include
--infodir=/host-native/usr/share/info --mandir=/host-native/usr/share/man
--disable-silent-rules --disable-dependency-tracking
--with-libtool-sysroot=/host-native --enable-clocale=generic --with-gnu-ld
--enable-shared --enable-languages=c,c++ --enable-threads=posix
--disable-multilib --enable-default-pie --enable-c99 --enable-long-long
--enable-symvers=gnu --enable-libstdcxx-pch
--program-prefix=arm-poky-linux-gnueabi- --without-local-prefix
--disable-install-libiberty --enable-lto --disable-libssp --enable-libitm
--disable-bootstrap --disable-libmudflap --with-system-zlib
--with-linker-hash-style=sysv --enable-linker-build-id --with-ppl=no
--with-cloog=no --enable-checking=release --enable-cheaders=c_global
--without-isl --with-gxx-include-dir=/not/exist/usr/include/c++/9.3.0
--with-sysroot=/not/exist --with-build-sysroot=/host
--enable-poison-system-directories --with-system-zlib --disable-static
--disable-nls --with-glibc-version=2.28 --enable-initfini-array
Thread model: posix
gcc version 9.3.0 (GCC)


<< code to demonstrate problem >>
typedef struct
{
  void (*request)(int request, void *arg);
} po_t;

po_t *po;
unsigned char *ambp(void);
unsigned char da;
unsigned char dic;

unsigned char gnda(unsigned char current_address, int sequence);
void rnsm(void);
int state = 0;
static unsigned char *attach_mb_ptr;
static unsigned char ta;
static unsigned char lda;

static unsigned char snod(unsigned char cd, unsigned char *tl, unsigned char
*ic)
{
   unsigned char nd;
   while ((nd = gnda(cd,0)) != 0x21)
   {
      int Req;
      po->request(57, &Req);
      if (!Req)
         break;
   }
   if (nd != 0x21)
   {
      switch (nd)
      {
         case 0x95:
            *tl = 0x01;
            break;

         case 0x80:
         case 0x81:
         case 0x82:
            *tl = 0x00;
            break;

         default:
            *tl = 0x05;
            break;
      }
   }
   if (nd != 0x21)
   {
      switch (nd)
      {
         case 0x95:
             *ic = nd - 0x91;
            break;

         case 0x80:
         case 0x81:
         case 0x82:
            *ic = nd - 0x80;    /* compiles into an unconditional store of 0x80
*/
            break;

         default:
            *ic = 0;
            break;
      }
   }
   return nd;
}

void rosm3(unsigned char reason)
{
   switch(state)
   {
      case 0:
           attach_mb_ptr = ambp();
           da = snod(0x20, &ta, &dic);
           break;
      case 1:
           lda = da;
           if ((da = snod(lda, &ta, &dic)) != 0x21)
              if (reason == 1)
                 rnsm();
           break;
   }
}

^ permalink raw reply	[flat|nested] 35+ messages in thread
end of thread, other threads:[~2023-09-19 11:26 UTC | newest]

Thread overview: 35+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-10-19 17:45 [Bug c/102844] New: gcc-9.x miscompiles unsigned char expression in switch breiten at lexmark dot com
2021-10-19 17:58 ` [Bug tree-optimization/102844] [9/10/11/12 Regression] " pinskia at gcc dot gnu.org
2021-10-19 18:11 ` pinskia at gcc dot gnu.org
2021-10-19 18:13 ` pinskia at gcc dot gnu.org
2021-10-19 18:23 ` [Bug tree-optimization/102844] [9/10/11/12 Regression] vrp inserting assert causes miscompiling when doing update_ssa pinskia at gcc dot gnu.org
2021-10-20  7:10 ` rguenth at gcc dot gnu.org
2021-10-20  7:44 ` rguenth at gcc dot gnu.org
2021-10-20  7:49 ` rguenth at gcc dot gnu.org
2021-10-20  8:08 ` marxin at gcc dot gnu.org
2021-10-20  8:28 ` rguenth at gcc dot gnu.org
2021-10-20  8:33 ` marxin at gcc dot gnu.org
2021-10-20  8:59 ` rguenth at gcc dot gnu.org
2021-10-20  9:00 ` rguenth at gcc dot gnu.org
2021-10-20  9:03 ` rguenth at gcc dot gnu.org
2021-10-20  9:08 ` rguenth at gcc dot gnu.org
2021-10-20  9:19 ` rguenth at gcc dot gnu.org
2021-10-20  9:21 ` aldyh at gcc dot gnu.org
2021-10-20  9:30 ` rguenther at suse dot de
2021-10-20  9:47 ` aldyh at gcc dot gnu.org
2021-10-20  9:50 ` rguenth at gcc dot gnu.org
2021-10-20  9:50 ` rguenth at gcc dot gnu.org
2021-10-20 10:56 ` marxin at gcc dot gnu.org
2021-10-20 12:49 ` [Bug tree-optimization/102844] [9/10/11/12 Regression] DOM jump threading not copying block that became non-empty amacleod at redhat dot com
2021-10-20 16:13 ` law at gcc dot gnu.org
2021-10-21  6:02 ` rguenth at gcc dot gnu.org
2022-05-20  8:24 ` [Bug tree-optimization/102844] [9/10/11/12/13 " rguenth at gcc dot gnu.org
2022-05-20 12:01 ` aldyh at gcc dot gnu.org
2022-05-20 15:11 ` law at gcc dot gnu.org
2022-05-20 15:33 ` aldyh at gcc dot gnu.org
2022-05-27  9:46 ` [Bug tree-optimization/102844] [10/11/12/13 " rguenth at gcc dot gnu.org
2022-06-28 10:46 ` jakub at gcc dot gnu.org
2023-01-19 13:26 ` rguenth at gcc dot gnu.org
2023-07-07 10:41 ` [Bug tree-optimization/102844] [11/12/13/14 " rguenth at gcc dot gnu.org
2023-09-19 11:24 ` rguenth at gcc dot gnu.org
2023-09-19 11:25 ` [Bug tree-optimization/102844] [11/12/13 " rguenth at gcc dot gnu.org

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