[Bug c/90267] New: [7.3 regression] wrong code generated wth -O2 as missing data dependence base on memory

[zhongyunde at huawei dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90267

            Bug ID: 90267
           Summary: [7.3 regression] wrong code generated wth -O2 as
                    missing data dependence base on memory
           Product: gcc
           Version: 7.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhongyunde at huawei dot com
  Target Milestone: ---

Created attachment 46254
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46254&action=edit
a simple testcase

test case attached, and we can see the code in line 42 clear the memory point
with pstruDlTpcPara, so the value in the memory cross the line 42 is different.

 39             TpcFingerNum[TpcIdex][0] = *(UINT32*)(void*)pstruDlTpcPara;
 40             TpcFingerNum[TpcIdex][1] = pstruDlTpcPara->ucThreeSlotFngNum;
 41 
 42             *(volatile UINT32*)(void*)pstruDlTpcPara = 0;
 43 
 44             TpcFingerNum[TpcIdex][2] = *(UINT32*)(void*)pstruDlTpcPara;
 45             TpcFingerNum[TpcIdex][3] = pstruDlTpcPara->ucThreeSlotFngNum;


base on the gcc 7.3, in the dumpfile test.c.037t.fre1, we can see the value _6
get before the operator of clear the memory. So it miss the data dependence
base on memory.
(Generated by cmd:../../GCC/gcc-730/binary/bin/gcc test.c -dAp -S -O2 -g3
-fdump-tree-all)

 37   _4 = MEM[(struct STRU_CCH_DLTPC_PARA
*)&gastruDlTpcPara][_1].ucThreeSlotFngNum;
 38   _6 = (unsigned int) _4;
 39   TpcFingerNum[TpcIdex.0_2][1] = _6;
 40   MEM[(volatile UINT32 *)pstruDlTpcPara_20] ={v} 0;
 41   _8 = MEM[(UINT32 *)pstruDlTpcPara_20];
 42   TpcFingerNum[TpcIdex.0_2][2] = _8;
 43   TpcFingerNum[TpcIdex.0_2][3] = _6;