Differential D21642
Remove more unused identifiers from r351198. Authored by markj on Sep 13 2019, 6:01 PM. Tags None Referenced Files
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Jeff pointed out that I missed these in r351742. Also sprinkle #ifdef __amd64__ vm_page_t const vm_page_array = (vm_page_t)VM_MIN_KERNEL_ADDRESS; #else vm_page_t vm_page_array; #endif But I don't really like having the inconsistent definition. I will
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If you start down this path, I would go a step further: Declare vm_page_array as an array, instead of a pointer, and define the symbol's value to VM_MIN_KERNEL_ADDRESS. This would eliminate the memory dereference to retrieve the start of the array. Comment Actions Sorry, I don't quite see what you are proposing. My suggestion also eliminated the memory dereference, but I don't see how declaring vm_page_array as an array would help anything. Comment Actions I'm afraid not. Consider this simplified example. const int *ptr = (int *)0x10000000;
int
func(void)
{
return (*ptr);
}Compiled with cc -O2 -S foo4.c to produce: .text
.file "foo4.c"
.globl func # -- Begin function func
.p2align 4, 0x90
.type func,@function
func: # @func
.cfi_startproc
# %bb.0: # %entry
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset %rbp, -16
movq %rsp, %rbp
.cfi_def_cfa_register %rbp
movq ptr(%rip), %rax
movl (%rax), %eax
popq %rbp
.cfi_def_cfa %rsp, 8
retq
.Lfunc_end0:
.size func, .Lfunc_end0-func
.cfi_endproc
# -- End function
.type ptr,@object # @ptr
.data
.globl ptr
.p2align 3
ptr:
.quad 268435456
.size ptr, 8
.ident "FreeBSD clang version 8.0.1 (tags/RELEASE_801/final 366581) (based on LLVM 8.0.1)"
.section ".note.GNU-stack","",@progbits
.addrsigComment Actions My proposal would be equivalent to int *const ptr = (int *)0x10000000, which yields func: # @func
.cfi_startproc
# %bb.0: # %entry
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset %rbp, -16
movq %rsp, %rbp
.cfi_def_cfa_register %rbp
movl 268435456, %eax
popq %rbp
.cfi_def_cfa %rsp, 8
retq
.Lfunc_end0:
.size func, .Lfunc_end0-func
.cfi_endprocComment Actions Yes, I put the const in the wrong place. So, code within vm_page.c will avoid the dereference, but code outside vm_page.c that uses the declaration from vm_page.h will still perform the dereference. Declaring vm_page_array as an array would deal with both cases. Comment Actions I'm still missing something. vm_page_array cannot be defined in vm_page.h, and if it is not defined there I do not see how the compiler can elide a memory dereference. I also do not see how an array symbol can be initialized to VM_MIN_KERNEL_ADDRESS. Comment Actions It cannot be defined, but can be declared, same as now. Or I do not understand what you are trying to say.
You use .set in some asm file. No C definition is provided. Comment Actions In other words, the following. Please do note the comment at the bottom. #define make_string(s) #s
#define value_to_string(v) make_string(v)
#define VM_MIN_KERNEL_ADDRESS 0x10000000
struct vm_page {
int dummy;
};
/*
* Replace the declaration
*
* extern vm_page_t vm_page_array;
*
* in vm/vm_page.h with the following on amd64:
*/
extern struct vm_page vm_page_array[];
/*
* Then, on amd64, replace the definition in vm/vm_page.c with the following
* in one .c or .s file:
*/
__asm (".set vm_page_array, " value_to_string(VM_MIN_KERNEL_ADDRESS));
int
main(void)
{
return (vm_page_array[0].dummy);
}
/*
* Which compiles (and links) to:
*
00000000002012d0 <main>:
2012d0: 55 push %rbp
2012d1: 48 89 e5 mov %rsp,%rbp
2012d4: 8b 05 26 ed df 0f mov 0xfdfed26(%rip),%eax # 10000000 <vm_page_array>
2012da: 5d pop %rbp
2012db: c3 retq
2012dc: cc int3
2012dd: cc int3
2012de: cc int3
2012df: cc int3
*
* The trouble is that vm_page_array won't be in the top 2GB of the address
* space, so the compiler would have to be convinced to use a different
* addressing mode.
*/Comment Actions Indeed, I cannot see a way to make the suggestion work without changing the code model for all files which reference vm_page_array. There is no "model" attribute for variables. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||