Description
| Bugzilla Link | 49600 |
| Version | trunk |
| OS | All |
| CC | @topperc,@RKSimon,@phoebewang,@rotateright |
Extended Description
Given a function as such
; RUN: llc < %s
define void @big_stack() "probe-stack"="inline-asm" uwtable {
start:
%_two_page_stack = alloca [8192 x i8], align 1
ret void
}
the following assembly will be generated:
big_stack:
.cfi_startproc
subq $4096, %rsp
movq $0, (%rsp)
subq $3968, %rsp
.cfi_def_cfa_offset 8072
addq $8064, %rsp
.cfi_def_cfa_offset 8
retq
Here the unwind tables are not accurate while stack probing is ongoing – the rsp is adjusted, but not the cfa_offsets. And so attempts to obtain a stack trace will fail if the current instruction is somewhere in between the instructions implementing the stack probing.
This also occurs with the non-unrolled implementation of the stack probing:
; RUN: llc < %s
define void @big_stack() "probe-stack"="inline-asm" uwtable {
start:
%_two_page_stack = alloca [64000 x i8], align 1
ret void
}
--->
big_stack:
.cfi_startproc
movq %rsp, %r11
subq $61440, %r11
.LBB0_1:
subq $4096, %rsp
movq $0, (%rsp)
cmpq %r11, %rsp
jne .LBB0_1
subq $2432, %rsp
.cfi_def_cfa_offset 63880
addq $63872, %rsp
.cfi_def_cfa_offset 8
retq
however in the loop case the solution needs to involve allocation of a separate register as insertion of .cfi directives in a loop won't help in any way.