On Mon, Oct 5, 2015 at 1:17 AM, H.J. Lu <hjl.to...@gmail.com> wrote: >>>>>> Looking a bit deeper into the code, it looks that we want to realign >>>>>> the stack in the interrupt handler. Let's assume that interrupt >>>>>> handler is calling some other function that saves SSE vector regs to >>>>>> the stack. According to the x86 ABI, incoming stack of the called >>>>>> function is assumed to be aligned to 16 bytes. But, interrupt handler >>>>>> violates this assumption, since the stack could be aligned to only 4 >>>>>> bytes for 32bit and 8 bytes for 64bit targets. Entering the called >>>>>> function with stack, aligned to less than 16 bytes will certainly >>>>>> violate ABI. >>>>>> >>>>>> So, it looks to me that we need to realign the stack in the interrupt >>>>>> handler unconditionally to 16bytes. In this case, we also won't need >>>>>> the following changes: >>>>>> >>>>> >>>>> Current stack alignment implementation requires at least >>>>> one, maybe two, scratch registers: >>>>> >>>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67841 >>>>> >>>>> Extend it to the interrupt handler, which doesn't have any scratch >>>>> registers may require significant changes in backend as well as >>>>> register allocator. >>>> >>>> But without realignment, the handler is unusable for anything but >>>> simple functions. The handler will crash when called function will try >>>> to save vector reg to stack. >>>> >>> >>> We can use unaligned load and store to avoid crash. >> >> Oh, sorry, I meant "called function will crash", like: >> >> -> interrupt when %rsp = 0x...8 -> >> -> interrupt handler -> >> -> calls some function that tries to save xmm reg to stack >> -> crash in the called function >> > > It should be fixed by this patch. But we need to fix stack > alignment in interrupt handler to avoid scratch register. > > > -- > H.J. > --- > commit 15f48be1dc7ff48207927d0b835e593d058f695b > Author: H.J. Lu <hjl.to...@gmail.com> > Date: Sun Oct 4 16:14:03 2015 -0700 > > Correctly set incoming stack boundary for interrupt handler > > diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c > index 7ebdcd9..0f0cc3c 100644 > --- a/gcc/config/i386/i386.c > +++ b/gcc/config/i386/i386.c > @@ -12037,8 +12037,11 @@ ix86_minimum_incoming_stack_boundary (bool sibcall) > { > unsigned int incoming_stack_boundary; > > + /* Stack of interrupt handler is always aligned to word_mode. */ > + if (cfun->machine->func_type != TYPE_NORMAL) > + incoming_stack_boundary = TARGET_64BIT ? 64 : 32;
Just a heads up that in order to support stack realignmnent on x86_64, MIN_STACK_BOUNDARY will soon be changed to BITS_PER_WORD, so you can use it in the line above. Please see comment #5 and #6 of PR 66697 [1]. > /* Prefer the one specified at command line. */ > - if (ix86_user_incoming_stack_boundary) > + else if (ix86_user_incoming_stack_boundary) > incoming_stack_boundary = ix86_user_incoming_stack_boundary; > /* In 32bit, use MIN_STACK_BOUNDARY for incoming stack boundary > if -mstackrealign is used, it isn't used for sibcall check and [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66697 Uros.
