On Wed, 8 Oct 2014, Uros Bizjak wrote: > Hello! > > This message revives an old thread [1], where the miscompilation of gfortran > on alpha was found that that resulted in: > > FAIL: gfortran.dg/assumed_rank_3.f90:15.20: > > print *, ubound(x,dim=3) ! << wrong dim > 1 > Error: Assumed-rank variable x at (1) may only be used as actual argument > > The problem was in the miscompilation of resolve_actual_arglist from > resolve.c. This function initializes two nearby global bool variables > with the following sequence: > > actual_arg = true; > first_actual_arg = true; > > but due to the miscompilation, the actual_arg was never set to true. > This happened due to the way stores to QImode and HImode locations are > implemented on non-BWX targets. The sequence reads full word, does its > magic to the part and stores the full word with changed part back to > the memory. However - the scheduler mixed two sequences, violating the > atomicity of RMW sequence. > > As demostrated in the great detail in the PR [2], the problem is in > early exit for MEM_READOLNY_P in true_dependence_1 in alias.c. This > early exit declares all MEM_READONLY_P references as non-aliasing, > which is not true when possibly aliasing references with alignment > ANDs are involved. > > Proposed patch prevents MEM_READONLY_P memory references to be moved > over possibly aliased memory (with alignment ANDs). The patch prevents > early exit for MEM_READONLY_P references when alignment ANDs are > involved. The aliasing is then determined later in the function. In > effect, it changes early exit to: > > - if (MEM_READONLY_P (x)) > - return 0; > + if (MEM_READONLY_P (x) > + && GET_CODE (x_addr) != AND > + && GET_CODE (mem_addr) != AND) > + return 0; > > The comment also mentions "... We don't expect to find read-only set > on MEM, but stupid user tricks can produce them, so don't die.". We > certainly don't die anymore, as confirmed by a native alpha-linux-gnu > (please note - not alphaev6!) bootstrap and regression test. > > 2014-10-08 Uros Bizjak <ubiz...@gmail.com> > > * alias.c (true_dependence_1): Do not exit early for MEM_READONLY_P > references when alignment ANDs are involved. > > The patch was bootstrapped and regression tested on alpha-linux-gnu. > > OK for mainline and release branches?
As said in the audit trail of the bugreport I think that the caller of alpha_set_memflags is wrong in applying MEM flags from the _source_ operand to MEMs generated for the RMW sequence for the destination. It would be better to fix that instead. Richard. > Please note that the patch by itself is not enough to fix the original > problem with gfortran miscompilation. Another problem in this area is > summarised in PR 63475 [3], where postreload CSE propagates aliased > memory operand. > > [1] https://gcc.gnu.org/ml/gcc-patches/2014-06/msg02251.html > [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63483 > [3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63475 > > Uros. > -- Richard Biener <rguent...@suse.de> SUSE / SUSE Labs SUSE LINUX Products GmbH - Nuernberg - AG Nuernberg - HRB 16746 GF: Jeff Hawn, Jennifer Guild, Felix Imend"orffer
