[Bug tree-optimization/100051] missed optimization for dead code elimination at -Os, -O2 and -O3 (vs. -O1)

[1364894213 at qq dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100051

huadong <1364894213 at qq dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |1364894213 at qq dot com

--- Comment #4 from huadong <1364894213 at qq dot com> ---
Hi, commit d1d01a6 has introduced a issue in gcc 12 on aarch64 platform.

1、 demo code:
 #include <stdio.h>
 #include <stdint.h>
 #include <stdbool.h>
 #include <ctype.h>

 #define reg_write(addr, val) \
     (*(volatile uint32_t *)(uintptr_t)(addr) = (val))


 static inline void writeb(void *addr, unsigned int val)
 {
     reg_write((uintptr_t)addr & (~0x3), val);
 }

 static char c;
 static void str_test(char *str)
 {
     c = str[1];
     if (islower(c))
         writeb(&c, toupper(c));
     putchar(c);
     putchar('\n');
 }


 int main(int argc, char *argv[])
 {
     if (argc > 1)
         str_test(argv[1]);
     else
         str_test(argv[0]);

     return 0;
 }

2、compile command:aarch64-linux-musl-gcc -O2 -fdump-tree-fre demo.c
dump gimple code:
void str_test (char * str)
 {
   char _1;
   unsigned int _3;
   unsigned int _4;
   int _6;
   int _7;
   unsigned int _8;
   int _10;
   long unsigned int addr.3_18;
   long unsigned int _19;
   volatile uint32_t * _20;

   <bb 2> :
   _1 = MEM[(char *)str_13(D) + 1B];
   c = _1;
   _3 = (unsigned int) _1;
   _4 = _3 + 4294967199;
   if (_4 <= 25)
     goto <bb 3>; [INV]
   else
     goto <bb 4>; [INV]

   <bb 3> :
   _6 = (int) _1;
   _7 = toupper (_6);
   _8 = (unsigned int) _7;
   addr.3_18 = (long unsigned int) &c;
   _19 = addr.3_18 & 18446744073709551612;
   _20 = (volatile uint32_t *) _19;
   *_20 ={v} _8;

   <bb 4> :
   _10 = (int) _1;
   putchar (_10);
   putchar (10);
   return;

 }

the character print by putchar should be uppercased _8, but it take original
value _1.