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From: "luke.geeson at cs dot ucl.ac.uk" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/111235] [Armv7-a]: Control-dependency between atomic
 accesses removed by -O1.
Date: Wed, 25 Oct 2023 16:10:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111235

--- Comment #5 from Luke Geeson <luke.geeson at cs dot ucl.ac.uk> ---
Thank you for fixing this, Wilco! I will now test this bug using the code
emitted by Godbolt.=20

Consider again the same source program. When run through gcc with the same
flags we get the ARM assembly test:
```
ARM test

{ [P0_r0]=3D0;[P1_r0]=3D0;[x]=3D0;[y]=3D0;
  %P0_P0_r0=3DP0_r0;%P0_x=3Dx;%P0_y=3Dy;
  %P1_P1_r0=3DP1_r0;%P1_x=3Dx;%P1_y=3Dy }
(*****************************************************************)
(*                 the Telechat toolsuite                        *)
(*                                                               *)
(* Luke Geeson, University College London, UK.                   *)
(* gcc -O1 -march=3Darmv7-a -pthread --std=3Dc11 -marm -fno-section-anchors=
*)
(*                                                               *)
(*****************************************************************)
  P0                   |  P1                   ;
   LDR R2,[%P0_x]      |   LDR R2,[%P1_y]      ;
   CMP R2,#1           |   DMB ISH             ;
   BEQ L3              |   MOV R1,#1           ;
L2: STR R2, [%P0_P0_r0]|   STR R1, [%P1_x]     ;
   BX LR               |   STR R2, [%P1_P1_r0] ;
L3: MOV R1, #1         |                       ;
   STR R1, [%P0_y]     |                       ;
   B L2                |                       ;


exists (P0_r0=3D1 /\ P1_r0=3D1)
```
Which under the arm model has the outcomes:
```
[P0_r0]=3D0; [P1_r0]=3D0;
[P0_r0]=3D1; [P1_r0]=3D0;
```

As you can see the buggy behaviour has gone away, the bug is fixed.=