R_386_GOT32 和 R_386_GOT32X 有什么区别？
What's the difference between R_386_GOT32 and R_386_GOT32X?
我找到了很多关于 R_386_GOT32 的文档，但没有找到 R_386_GOT32X。我能够找到显示 R_386_GOT32X 计算的 https://github.com/hjl-tools/x86-psABI/wiki/intel386-psABI-draft.pdf，但它看起来与 R_386_GOT32
的计算相同
R_386_GOT32 | 3 | word32 | G + A - GOT / G + A†
R_386_GOT32X | 43 | word32 | G + A - GOT / G + A†
两种重定位具有相同的效果并且计算相同，但是R_386_GOT32X重定位允许链接器优化用于计算的指令，在某些条件下使用立即操作数而不是内存操作数。
H. J. Lu（介绍 R_386_GOT32X 的英特尔工程师）explains it 在 IA32 System V ABI 邮件列表中：
X86 instruction encoding supports converting some instructions on
memory operand with GOT32 relocation against symbol, foo, into a
different form on immediate operand if foo is defined locally. Those
instructions are:
call *foo@GOT[(%reg)] => nop call foo or call foo nop
jmp *foo@GOT[(%reg)] => jmp foo nop
mov foo@GOT[(%reg1)], %reg2 => lea foo[@GOTOFF(%reg1)], %reg2
When osition-independent code is disable,
test %reg1, foo@GOT[(%reg2)] => test $foo, %reg1 binop
foo@GOT[(%reg1)], %reg2 => binop $foo, %reg2
where binop is one of adc, add, and, cmp, or, sbb, sub, xor
instructions.
I am proposing to add a new relocation, R_386_GOT32X, to i386 psABI.
Instead of generating R_386_GOT32 relocation agasint foo for
foo@GOT(%reg), we generate R_386_GOT32X. R_386_GOT32X relocation can
also be used without the base register for the global offset table,
foo@GOT, when position-independent code is disable. In this case, the
static base address of the global offset table will be used instead.
Linker can treat R_386_GOT32X the same as R_386_GOT32 or it can
perform the transformations listed above.
确实，根据您链接的 IA32 System V ABI 文档（A.2 节）：
Optimize R_386_GOT32X Relocation
The Intel386 instruction encoding supports converting certain
instructions on memory operand with R_386_GOT32X relocation against
symbol, foo, into a different form on immediate operand if foo is
defined locally:
Convert call, jmp and mov Convert memory operand
of call, jmp and mov into immediate operand.
Memory Operand
Immediate Operand
call *foo@GOT(%reg)nop call foo
call *foo@GOT(%reg)call foo nop
jmp *foo@GOT(%reg)jmp foo nop
mov foo@GOT(%reg1), %reg2lea foo@GOTOFF(%reg1), %reg2
Convert Test and Binop Convert memory operand of call, jmp, mov, test
and binop into immediate operand, where binop is one of adc, add,
and, cmp, or, sbb, sub, xor instructions, when position-independent
code is disabled.
Memory Operand
Immediate Operand
call *foo@GOTnop call foo
call *foo@GOTcall foo nop
jmp *foo@GOTjmp foo nop
mov foo@GOT, %regmov $foo, %reg
test %reg, foo@GOTtest $foo, %reg
binop foo@GOT, %regbinop $foo, %reg
call *foo@GOT(%reg)nop call foo
call *foo@GOT(%reg)call foo nop
jmp *foo@GOT(%reg)jmp foo nop
mov foo@GOT(%reg1), %reg2mov $foo, %reg2
test %reg1, name@GOT(%reg2)test $foo, %reg1
binop name@GOT(%reg1), %reg2binop $foo, %reg2
我找到了很多关于 R_386_GOT32 的文档，但没有找到 R_386_GOT32X。我能够找到显示 R_386_GOT32X 计算的 https://github.com/hjl-tools/x86-psABI/wiki/intel386-psABI-draft.pdf，但它看起来与 R_386_GOT32
R_386_GOT32 | 3 | word32 | G + A - GOT / G + A†
R_386_GOT32X | 43 | word32 | G + A - GOT / G + A†
两种重定位具有相同的效果并且计算相同，但是R_386_GOT32X重定位允许链接器优化用于计算的指令，在某些条件下使用立即操作数而不是内存操作数。
H. J. Lu（介绍 R_386_GOT32X 的英特尔工程师）explains it 在 IA32 System V ABI 邮件列表中：
X86 instruction encoding supports converting some instructions on memory operand with GOT32 relocation against symbol, foo, into a different form on immediate operand if foo is defined locally. Those instructions are:
call *foo@GOT[(%reg)] => nop call foo or call foo nop jmp *foo@GOT[(%reg)] => jmp foo nop mov foo@GOT[(%reg1)], %reg2 => lea foo[@GOTOFF(%reg1)], %reg2When osition-independent code is disable,
test %reg1, foo@GOT[(%reg2)] => test $foo, %reg1 binop foo@GOT[(%reg1)], %reg2 => binop $foo, %reg2where binop is one of adc, add, and, cmp, or, sbb, sub, xor instructions.
I am proposing to add a new relocation, R_386_GOT32X, to i386 psABI. Instead of generating R_386_GOT32 relocation agasint foo for foo@GOT(%reg), we generate R_386_GOT32X. R_386_GOT32X relocation can also be used without the base register for the global offset table, foo@GOT, when position-independent code is disable. In this case, the static base address of the global offset table will be used instead. Linker can treat R_386_GOT32X the same as R_386_GOT32 or it can perform the transformations listed above.
确实，根据您链接的 IA32 System V ABI 文档（A.2 节）：
Optimize R_386_GOT32X Relocation
The Intel386 instruction encoding supports converting certain instructions on memory operand with
R_386_GOT32Xrelocation against symbol,foo, into a different form on immediate operand iffoois defined locally:Convert call, jmp and mov Convert memory operand of
call,jmpandmovinto immediate operand.
Memory Operand Immediate Operand call *foo@GOT(%reg)nop call foocall *foo@GOT(%reg)call foo nopjmp *foo@GOT(%reg)jmp foo nopmov foo@GOT(%reg1), %reg2lea foo@GOTOFF(%reg1), %reg2Convert Test and Binop Convert memory operand of
call,jmp,mov,testandbinopinto immediate operand, wherebinopis one ofadc,add,and,cmp,or,sbb,sub,xorinstructions, when position-independent code is disabled.
Memory Operand Immediate Operand call *foo@GOTnop call foocall *foo@GOTcall foo nopjmp *foo@GOTjmp foo nopmov foo@GOT, %regmov $foo, %regtest %reg, foo@GOTtest $foo, %regbinop foo@GOT, %regbinop $foo, %regcall *foo@GOT(%reg)nop call foocall *foo@GOT(%reg)call foo nopjmp *foo@GOT(%reg)jmp foo nopmov foo@GOT(%reg1), %reg2mov $foo, %reg2test %reg1, name@GOT(%reg2)test $foo, %reg1binop name@GOT(%reg1), %reg2binop $foo, %reg2