On Thu, Mar 22, 2018 at 4:56 AM, Renlin Li <renlin...@foss.arm.com> wrote: > Hi all, > > As described in PR84877. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84877 > The local copy of parameter on stack is not aligned. > > For BLKmode paramters, a local copy on the stack will be saved. > There are three cases: > 1) arguments passed partially on the stack, partially via registers. > 2) arguments passed fully on the stack. > 3) arguments passed via registers. > > After the change here, in all three cases, the stack slot for the local > parameter copy is aligned by the data type. > The stack slot is the DECL_RTL of the parameter. All the references > thereafter in the function will refer to this RTL. > > To populate the local copy on the stack, > For case 1) and 2), there are operations to move data from the caller's > stack (from incoming rtl) into callee's stack. > For case 3), the registers are directly saved into the stack slot. > > In all cases, the destination address is properly aligned. > But for case 1) and case 2), the source address is not aligned by the type. > It is defined by the PCS how the arguments are prepared. > The block move operation is fulfilled by emit_block_move (). As far as I can > see, > it will use the smaller alignment of source and destination. > This looks fine as long as we don't use instructions which requires a strict > larger alignment than the address actually has. > > Here, it only changes receiving parameters. > The function assign_stack_local_1 will be called in various places. > Usually, the caller will constraint the ALIGN parameter. For example via > STACK_SLOT_ALIGNMENT macro. > assign_parm_setup_block will call assign_stack_local () with alignment from > the parameter type which in this case could be > larger than MAX_SUPPORTED_STACK_ALIGNMENT. > > The alignment operation for parameter copy on the stack is similar to stack > vars. > First, enough space is reserved on the stack. The size is fixed at compile > time. > Instructions are emitted to dynamically get an aligned address at runtime > within this piece of memory. > > This will unavoidably increase the usage of stack. However, it really > depends on > how many over-aligned parameters are passed by value. > > x86-linux, arm-none-eabi, aarch64-one-elf regression test Okay. > linux-armhf bootstrap Okay. > > I assume there are other targets which will be affected by the change. > But I don't have environment to test. > > Okay the commit? > > > Regards, > Renlin > > gcc/ > > 2018-03-22 Renlin Li <renlin...@arm.com> > > PR middle-end/84877 > * explow.h (get_dynamic_stack_size): Declare it as external. > * explow.c (record_new_stack_level): Remove function static > attribute. > * function.c (assign_stack_local_1): Dynamically align the stack > slot > addr for parameter copy on the stack. >
Will this coded be used on x86? X86 backend can dynamically align stack just fine without the above change. -- H.J.
