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From: "stuwph at live dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/64491] New: warning: loop exit may only be reached after undefined behavior
Date: Sun, 04 Jan 2015 20:24:00 -0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 5.0
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64491

            Bug ID: 64491
           Summary: warning: loop exit may only be reached after undefined
                    behavior
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stuwph at live dot de

gcc -fno-exceptions -Wno-long-long -fno-rtti -ansi -pedantic -DNDEBUG -O3
-DIS_64BIT -DUSE_BSFQ -DUSE_PREFETCH -static -s


In function 'void Bitboards::init()': 
warning: loop exit may only be reached after undefined behavior
[-Waggressive-loop-optimizations] 
for (File f = FILE_A; f <= FILE_H; ++f) 
                        ^ 
note: possible undefined statement is here 
AdjacentFilesBB[f] = (f > FILE_A ? FileBB[f - 1] : 0) | (f < FILE_H ? FileBB[f
+ 1] : 0);                                            ^



/// Bitboards::init() initializes various bitboard tables. It is called at
/// startup and relies on global objects to be already zero-initialized.

void Bitboards::init() {

  for (Square s = SQ_A1; s <= SQ_H8; ++s)
  {
      SquareBB[s] = 1ULL << s;
      BSFTable[bsf_index(SquareBB[s])] = s;
  }

  for (Bitboard b = 1; b < 256; ++b)
      MS1BTable[b] = more_than_one(b) ? MS1BTable[b - 1] : lsb(b);

  for (File f = FILE_A; f <= FILE_H; ++f)
      FileBB[f] = f > FILE_A ? FileBB[f - 1] << 1 : FileABB;

  for (Rank r = RANK_1; r <= RANK_8; ++r)
      RankBB[r] = r > RANK_1 ? RankBB[r - 1] << 8 : Rank1BB;


/// warning here:

  for (File f = FILE_A; f <= FILE_H; ++f)
      AdjacentFilesBB[f] = (f > FILE_A ? FileBB[f - 1] : 0) | (f < FILE_H ?
FileBB[f] : 0);

  for (Rank r = RANK_1; r < RANK_8; ++r)
      InFrontBB[WHITE][r] = ~(InFrontBB[BLACK][r + 1] = InFrontBB[BLACK][r] |
RankBB[r]);

...


*** definitely it is a gcc bug, the warning is wrong.

My guess is that gcc consider that FileBB[f + 1] could reach out of bound when
f == FILE_H, ignoring that out of bound access is guarded by the condition (f <
FILE_H)


*** solution to let the warning disappear:

for (File f = FILE_A; f <= FILE_H; ++f)
AdjacentFilesBB[f] = (f > FILE_A ? FileBB[f - 1] : 0) | (f < FILE_H ? FileBB[f]
: 0);