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From: "thopre01 at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/64458] New: [ARM] Redundant ldr when accessing var inside and outside a loop
Date: Wed, 31 Dec 2014 16:33:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64458

            Bug ID: 64458
           Summary: [ARM] Redundant ldr when accessing var inside and
                    outside a loop
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thopre01 at gcc dot gnu.org
            Target: arm-none-eabi

When compiling the below example with -O2, 2 ldr are generated to access the
variable "some": one for the loop and one for the store.

int f (int);

unsigned int glob;

void
g (void)
{
  while (glob);
  glob = 1;
}

The following code is then generated:

0: push
{r4, lr}
4: ldr r4, pc, #32 ; 2c
8: ldr r0, [r4]
c: bl 0 <g>
10: cmp r0, #0
14: ldr r3, pc, #16 ; 2c
18: bne 8
1c: mov r2, #1
20: pop {r4, lr}

24: str r2, [r3]
28: bx lr
2c: .word 0x00000000

Note the redundant load of the address at offset 2c in instructions at offsets
4 and 14.