[Bug c/83510] New: Recent changes for -Warray-bounds trigger false positive

[sirl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83510

            Bug ID: 83510
           Summary: Recent changes for -Warray-bounds trigger false
                    positive
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sirl at gcc dot gnu.org
  Target Milestone: ---

Created attachment 42933
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42933&action=edit
testcase

The attached testcase started to produce a warning with trunk somewhere between
r255501 and r255678, with -O2 -Warray-bounds:

test-Warray-bounds.c: In function 'g':
test-Warray-bounds.c:18:13: warning: array subscript 4294967286 is above array
bounds of 'unsigned int[10]' [-Warray-bounds=]
   return arr[number - 0xa];
          ~~~^~~~~~~~~~~~~~

I believe it is a false positive.


unsigned int arr[10];

struct xyz {
        unsigned int a0;
};

extern void wfm(struct xyz *, int, unsigned int);

static unsigned int f(struct xyz * ctx, unsigned int number)
{
        switch (number) {
        case 0x9:
                return ctx->a0;
        case 0xA: case 0xB:
        case 0xC: case 0xD: case 0xE: case 0xF:
        case 0x10: case 0x11: case 0x12: case 0x13:
                return arr[number - 0xa];
        }
        return 0;
}

int g(struct xyz * ctx) {
        int i;

        for (i = 0; i < 10; i++) {
                wfm(ctx, i, f(ctx, i));
        }

        return 0;
}