This bug was noticed on sh but potentially impacts other STRICT_ALIGNMENT
targets.
The attached test case reduces a misaligned field access from an array of
packed structs. In this example compiled with -O2 the field wMaxPacketSize is
accessed (on sh4) with a mov.w instruction although it is byte aligned.
It seems that the tree-ssa-loop ivopts did not check packed struct offsets
indexed by an induction variable. Thus if the struct size is not aligned, the
field becomes unaligned after the first iteration even if it was aligned from
the base of this structure.
The proposed patch solves this problem by expanding may_be_unaligned_p to check
that a loop carried offset is a multiple of the desired alignment.
--
Summary: Misaligned Accesses on arrays of packed stucts
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: chrbr at gcc dot gnu dot org
GCC target triplet: sh4-linux
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35178
