Created attachment 34953 [details] Test case When GCC has multiple local variables with different aligned attributes on them, it creates an aligned block of space aligned to the least aligned variable instead of the most aligned variable. In the attached program, when run on MIPS, GCC is creating a 32 byte aligned block of memory for foo1, foo3, and foo4 and a 128 byte aligned memory block for foo2. foo3 and foo4 should have 128 byte aligned memory. I thought the problem was in stack_var_cmp where it checks the alignment of two variables and returns '(int) largeb - (int) largea' to sort the variables based on their alignment but when I swapped the two arguments I got an ICE in expand_stack_vars [gcc_assert (large_base != NULL);]. It also tried changing: large_align = stack_vars[stack_vars_sorted[0]].alignb * BITS_PER_UNIT; to large_align = stack_vars[stack_vars_sorted[n-1]].alignb * BITS_PER_UNIT; but that caused the same ICE. It may be that we need a loop to check the alignment of each variable.
It might be easier to see this bug if you apply this patch: diff --git a/gcc/cfgexpand.c b/gcc/cfgexpand.c index 7dfe1f6..7beb00e 100644 --- a/gcc/cfgexpand.c +++ b/gcc/cfgexpand.c @@ -973,6 +973,8 @@ expand_stack_vars (bool (*pred) (size_t), struct sta ck_vars_data *data) i = stack_vars_sorted[si]; alignb = stack_vars[i].alignb; + gcc_assert (alignb*BITS_PER_UNIT <= large_align); + /* Stop when we get to the first decl with "small" alignment. */ if (alignb * BITS_PER_UNIT <= MAX_SUPPORTED_STACK_ALIGNMENT) break; This way you get an ICE instead of just incorrectly aligned vectors.
I submitted a proposed fix. https://gcc.gnu.org/ml/gcc-patches/2015-03/msg00244.html
Author: sje Date: Thu Mar 5 16:34:03 2015 New Revision: 221219 URL: https://gcc.gnu.org/viewcvs?rev=221219&root=gcc&view=rev Log: 2015-03-05 Steve Ellcey <sellcey@imgtec.com> PR middle-end/65315 * cfgexpand.c (expand_stack_vars): Update large_align to maximum needed alignment. Modified: trunk/gcc/ChangeLog trunk/gcc/cfgexpand.c
A patch for this was checked in before 5.0 branched.