'sp'在 asm clobber 列表中是什么意思？

Question

我正在努力获得一些x86汇编在Core 2 Duo Mac上编译。使用clang返回invalid operand错误，如以下语句:

asm volatile ("subl %0,%%esp" :: "r"(foobar) : "cc", "sp");

其中foobar为long。我想知道sp关键字在clober列表中意味着什么，因为GCC/llvm文档和谷歌都不能对它们透露太多信息。有人知道一个完整的关键词列表吗?

Answer 1

肯定是堆栈指针，特别是，因为您实际上从堆栈指针esp中减去了一些东西。您可能已经知道这一点，但是，以防万一，cc是条件代码寄存器(标志等)。

为什么是sp而不是esp，我不能告诉你，但是，如果你得到错误，这是我要改变的第一件事，看看这是不是问题。

你应该能够放在那里的值是所有可能被破坏的寄存器(除了那些显式列出的输出，gcc已经知道会改变)和特殊值memory，如果你不希望gcc假设任何缓存的内存值将保持有效。

当然，寄存器依赖于底层体系结构。不确定这是不是一个全面的列表，但这是我曾经使用过的，或者需要使用的。

Answer 2

查看gcc-4.4.3/gcc/config/i386/i386.h:2036文件，我得出结论，"sp"是"esp"的gcc特定别名:

/* How to refer to registers in assembler output.
   This sequence is indexed by compiler's hard-register-number (see above).  */
/* In order to refer to the first 8 regs as 32-bit regs, prefix an "e".
   For non floating point regs, the following are the HImode names.
   For float regs, the stack top is sometimes referred to as "%st(0)"
   instead of just "%st".  PRINT_OPERAND handles this with the "y" code.  */
#define HI_REGISTER_NAMES                                               
{"ax","dx","cx","bx","si","di","bp","sp",                               
 "st","st(1)","st(2)","st(3)","st(4)","st(5)","st(6)","st(7)",          
 "argp", "flags", "fpsr", "fpcr", "frame",                              
 "xmm0","xmm1","xmm2","xmm3","xmm4","xmm5","xmm6","xmm7",               
 "mm0", "mm1", "mm2", "mm3", "mm4", "mm5", "mm6", "mm7",                
 "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15",                  
 "xmm8", "xmm9", "xmm10", "xmm11", "xmm12", "xmm13", "xmm14", "xmm15"}
#define REGISTER_NAMES HI_REGISTER_NAMES
/* Table of additional register names to use in user input.  */
#define ADDITIONAL_REGISTER_NAMES 
{ { "eax", 0 }, { "edx", 1 }, { "ecx", 2 }, { "ebx", 3 },       
  { "esi", 4 }, { "edi", 5 }, { "ebp", 6 }, { "esp", 7 },       
  { "rax", 0 }, { "rdx", 1 }, { "rcx", 2 }, { "rbx", 3 },       
  { "rsi", 4 }, { "rdi", 5 }, { "rbp", 6 }, { "rsp", 7 },       
  { "al", 0 }, { "dl", 1 }, { "cl", 2 }, { "bl", 3 },           
  { "ah", 0 }, { "dh", 1 }, { "ch", 2 }, { "bh", 3 } }

"sp"one_answers"esp"(以及"rsp")都将对编号为7的寄存器进行编码。

所以，这段代码在GCC中是合法的;但是如果你想让它可移植(通过clang编译)，把"sp"改成"esp"。这个补丁不会改变gcc的逻辑，并且可以让你用clang来构建它。

= = = =

更新

有可能GCC也为clobbers存储寄存器访问的大小。下面是asm寄存器的检查器函数- clobbers冲突(gcc/stmt.c):

tree_conflicts_with_clobbers_p (tree t, HARD_REG_SET *clobbered_regs)
{
  /* Conflicts between asm-declared register variables and the clobber
     list are not allowed.  */
  tree overlap = tree_overlaps_hard_reg_set (t, clobbered_regs);

冲突(重叠)通过HARD_REG_SET检查，大小为52位:

gcc/hard-reg-set.h: 50。Hard_reg_set的长度为FIRST_PSEUDO位，四舍五入以完全填充fastint。

#define HARD_REG_SET_LONGS 
 ((FIRST_PSEUDO_REGISTER + HOST_BITS_PER_WIDEST_FAST_INT - 1)   
  / HOST_BITS_PER_WIDEST_FAST_INT)
typedef HARD_REG_ELT_TYPE HARD_REG_SET[HARD_REG_SET_LONGS];

i386/i386.h:865: #define FIRST_PSEUDO_REGISTER 53

:882同一个文件在HARD_REG_SET:

中有一个寄存器列表

#define FIXED_REGISTERS                                         
/*ax,dx,cx,bx,si,di,bp,sp,st,st1,st2,st3,st4,st5,st6,st7*/      
{  0, 0, 0, 0, 0, 0, 0, 1, 0,  0,  0,  0,  0,  0,  0,  0,       
/*arg,flags,fpsr,fpcr,frame*/                                   
    1,    1,   1,   1,    1,                                    
/*xmm0,xmm1,xmm2,xmm3,xmm4,xmm5,xmm6,xmm7*/                     
     0,   0,   0,   0,   0,   0,   0,   0,                      
/* mm0, mm1, mm2, mm3, mm4, mm5, mm6, mm7*/                     
     0,   0,   0,   0,   0,   0,   0,   0,                      
/*  r8,  r9, r10, r11, r12, r13, r14, r15*/                     
     2,   2,   2,   2,   2,   2,   2,   2,                      
/*xmm8,xmm9,xmm10,xmm11,xmm12,xmm13,xmm14,xmm15*/               
     2,   2,    2,    2,    2,    2,    2,    2 }

所以register- clober冲突检查没有大小字段检查(对于非mm/xmm regs)。