-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 04/20/11 13:06, Xinliang David Li wrote: > On Wed, Apr 20, 2011 at 12:03 PM, Cary Coutant <ccout...@google.com> wrote: >>> This brings out 2 questions. Why don't GCC 4.4/4.6/4.7 warn it? >>> Why doesn't 64bit GCC 4.2 warn it? >> >> Good question. It seems that the difference is whether the compiler >> generates a field-by-field copy or a call to memcpy(). According to >> David, the trunk gcc in 32-bit mode doesn't call memcpy, but still >> doesn't warn. He's looking at it. > > It seems to be related to alias rewrite -- the use of single memory > token somehow blocks the warning (even though the struct read are > SRAed). You can end up tripping over all kinds of things. In fact, I found myself today looking at this for -Warray-bounds:
# BLOCK 11 freq:4946 # PRED: 9 [50.0%] (false,exec) 10 [100.0%] (fallthru,exec) 8 [28.0%] (false,exec) Invalid sum of incoming frequencies 2819, should be 4946 # D.39048_1 = PHI <3(9), D.39048_19(10), 4294967295(8)> # VUSE <.MEM_38(D)> D.39016_24 = default_target_hard_regs.x_fixed_regs[D.39048_1]; Note the value of D.39041_1 if we enter through block #8. We get no warning. Now one of my local patches happens to isolate the path which includes the edge 8->11 creating this block: # BLOCK 18 # PRED: 8 [28.0%] (false,exec) Invalid sum of incoming frequencies 692, should be 0 # VUSE <.MEM_39(D)> D.38986_52 = default_target_hard_regs.x_fixed_regs[4294967295]; Which (of course) triggers the -Warray-bounds warning. Whee fun, optimize better and expose a warning. As it turns out the path including 8->11 is unexecutable, but to expose that we're really going to have to tackle path isolation in a real way rather than the ad-hoc stuff done by the jump threader right now. Jeff -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) Comment: Using GnuPG with Fedora - http://enigmail.mozdev.org/ iQEcBAEBAgAGBQJNr2X8AAoJEBRtltQi2kC7Bo8H/0Le7ZdG8Fi/XMRPqbc3oiF/ A8zipmNXNdw29JTH488GJR4BtAuhidiw1EmdUBgj4UxhNXVvHkF+zJBEnzh/6oS5 exY9qTU0/hY6eW4UsdefQPczu1Uy/CSJgs9jFzZjLku6ao6EudOdiE5xXS6xc4Iq K2epQkMLFigc05W7TNGWNFC+r6Ty6OlS1xz8IIUOunjBwTax4QWQknAy5YDvXiF+ yJCTvattorNxgCEYizG+sVCo0/wNtZB0s2V/TDCdOWeOsZI2/yWC8+QKGcacnZRQ L2vGQkq7fHScNxsLrxSGbnCj72W9StM/eSYU9ABfMl4KLcq2miuy7JXEz3qDlok= =LO3d -----END PGP SIGNATURE-----
