Arjan van de Ven <ar...@linux.intel.com> writes: > On 8/7/2017 8:43 AM, Jakub Jelinek wrote: >> On Mon, Aug 07, 2017 at 08:39:24AM -0700, H.J. Lu wrote: >>> When Linux/x86-64 kernel is compiled with -fno-omit-frame-pointer. >>> this optimization removes more than 730 >>> >>> pushq %rbp >>> movq %rsp, %rbp >>> popq %rbp >> >> If you don't want the frame pointer, why are you compiling with >> -fno-omit-frame-pointer? Are you going to add >> -fforce-no-omit-frame-pointer or something similar so that people can >> actually get what they are asking for? This doesn't really make sense. >> It is perfectly fine to omit frame pointer by default, when it isn't >> required for something, but if the user asks for it, we shouldn't ignore his >> request. >> > > > wanting a framepointer is very nice and desired... ... but if the > optimizer/ins scheduler moves instructions outside of the frame'd > portion, (it does it for cases like below as well), the value is > already negative for these functions that don't have stack use. > > <MPIDU_Sched_are_pending@@Base>: > mov all_schedules@@Base-0x38460,%rax > push %rbp > mov %rsp,%rbp > pop %rbp > cmpq $0x0,(%rax) > setne %al > movzbl %al,%eax > retq
Yeah, and it could be even weirder for big single-block functions. I think GCC has been doing this kind of scheduling of prologue and epilogue instructions for a while, so there hasn*t really been a guarantee which parts of the function will have a new FP and which will still have the old one. Also, with an arbitrarily-picked host compiler (GCC 6.3.1), shrink-wrapping kicks in when the following is compiled with -O3 -fno-omit-frame-pointer: void f (int *); void g (int *x) { for (int i = 0; i < 1000; ++i) x[i] += 1; if (x[0]) { int temp; f (&temp); } } so only the block with the call to f sets up FP. The relatively long-running loop runs with the caller's FP. I hope we can go for a target-independent position that what HJ*s patch does is OK... Thanks, Richard