[Bug optimization/14272] [tree-ssa] miscompilation of __exchange_and_add (atomicity.h)

[steven at gcc dot gnu dot org]

------- Additional Comments From steven at gcc dot gnu dot org  2004-02-25 13:43 -------
This is not _really_ a TER problem, but a problem with volatiles and aliasing: 
 
*************** 
*** 17,34 **** 
 
  Coalesced label blocks: 0 (Max so far: 0) 
 
! int __exchange_and_add(volatile int*, int) (__mem, __val) 
  { 
    int __tmp; 
    int __result; 
    int T.1; 
!   volatile int T.0; 
 
    # BLOCK 0 
    # PRED: ENTRY [100.0%]  (fallthru,exec) 
    __result_2 = *__mem_1; 
!   T.0_3 = *__mem_1; 
!   T.1_5 = T.0_3 + __val_4; 
    *__mem_1 = T.1_5; 
    return __result_2; 
    # SUCC: EXIT [100.0%] 
--- 17,35 ---- 
 
  Coalesced label blocks: 0 (Max so far: 0) 
 
! int __exchange_and_add(int*, int) (__mem, __val) 
  { 
    int __tmp; 
    int __result; 
    int T.1; 
!   int T.0; 
 
    # BLOCK 0 
    # PRED: ENTRY [100.0%]  (fallthru,exec) 
+   #   VUSE <TMT.2_7>; 
    __result_2 = *__mem_1; 
!   T.1_5 = __result_2 + __val_4; 
!   #   TMT.2_8 = VDEF <TMT.2_7>; 
    *__mem_1 = T.1_5; 
    return __result_2; 
    # SUCC: EXIT [100.0%] 
 
Note the missing VUSE in the dump where __mem is volatile. 
 

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14272