On Thu, May 12, 2016 at 12:17 PM, Richard Biener
<richard.guent...@gmail.com> wrote:
> On Thu, May 12, 2016 at 12:10 PM, Claudiu Zissulescu
> <claudiu.zissule...@synopsys.com> wrote:
>> Hi,
>>
>> I've been trying the following simple test case on latest gcc, and it seems
>> to produce unwanted unaligned accesses for bit-fields.
>>
>> Test cases:
>>
>> struct lock_chain {
>> unsigned int irq_context: 2,
>> depth: 6,
>> base: 24;
>> };
>>
>> struct lock_chain * foo (struct lock_chain *chain)
>> {
>> int i;
>> for (i = 0; i < 100; i++)
>> {
>> chain[i+1].base = chain[i].base;
>> }
>> return chain;
>> }
>>
>> GCC options -O3 (we need predictive commoning to kick in).
>>
>> The result for ARM:
>>
>> .cpu arm926ej-s
>> .eabi_attribute 20, 1
>> .eabi_attribute 21, 1
>> .eabi_attribute 23, 3
>> .eabi_attribute 24, 1
>> .eabi_attribute 25, 1
>> .eabi_attribute 26, 1
>> .eabi_attribute 30, 2
>> .eabi_attribute 34, 0
>> .eabi_attribute 18, 4
>> .file "t01.c"
>> .text
>> .align 2
>> .global foo
>> .syntax unified
>> .arm
>> .fpu softvfp
>> .type foo, %function
>> foo:
>> @ args = 0, pretend = 0, frame = 0
>> @ frame_needed = 0, uses_anonymous_args = 0
>> @ link register save eliminated.
>> ldr r1, [r0, #1] <<<<<<<<<<<< Unaligned access
>> add r3, r0, #4
>> lsl r1, r1, #8
>> add ip, r0, #404
>> .L2:
>> ldrb r2, [r3] @ zero_extendqisi2
>> orr r2, r1, r2
>> str r2, [r3], #4
>> cmp r3, ip
>> bne .L2
>> bx lr
>> .size foo, .-foo
>> .ident "GCC: (GNU) 7.0.0 20160502 (experimental)"
>>
>> Please observe the ldr instruction will access an unaligned address.
>>
>> I observed this issue for ARC processor, but because the ARM is more
>> popular, I've use it for this case.
>>
>> Is this expected behavior or do I miss something?
>
> Looks like a bug - please file a bugreport.
Try
Index: gcc/tree-predcom.c
===================================================================
--- gcc/tree-predcom.c (revision 236159)
+++ gcc/tree-predcom.c (working copy)
@@ -1391,9 +1395,10 @@ ref_at_iteration (data_reference_p dr, i
&& DECL_BIT_FIELD (TREE_OPERAND (DR_REF (dr), 1)))
{
tree field = TREE_OPERAND (DR_REF (dr), 1);
+ tree type = build_aligned_type (DECL_BIT_FIELD_TYPE (field),
+ BITS_PER_UNIT);
return build3 (BIT_FIELD_REF, TREE_TYPE (DR_REF (dr)),
- build2 (MEM_REF, DECL_BIT_FIELD_TYPE (field),
- addr, alias_ptr),
+ build2 (MEM_REF, type, addr, alias_ptr),
DECL_SIZE (field), bitsize_zero_node);
}
else
Richard.
> Richard.
>
>> Thank you,
>> Claudiu
