I've found a lot of documentation for R_386_GOT32
, but not R_386_GOT32X
. I was able to find https://github.com/hjl-tools/x86-psABI/wiki/intel386-psABI-draft.pdf which shows the calculation for R_386_GOT32X
, but it looks to be the same as that for R_386_GOT32
R_386_GOT32 | 3 | word32 | G + A - GOT / G + A†
R_386_GOT32X | 43 | word32 | G + A - GOT / G + A†
Both relocations have the same effect and the calculations are the same, but R_386_GOT32X
relocations allow the linker to optimize the instructions used for the calculation, using immediate operands instead of memory operands under certain conditions.
H. J. Lu (Intel engineer who introduced R_386_GOT32X
) explains it in the IA32 System V ABI mailing list:
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.
Indeed, according to the IA32 System V ABI document you linked (section A.2):
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 iffoo
is defined locally:Convert call, jmp and mov Convert memory operand of
call
,jmp
andmov
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), %reg2
lea foo@GOTOFF(%reg1), %reg2
Convert Test and Binop Convert memory operand of
call
,jmp
,mov
,test
andbinop
into immediate operand, wherebinop
is one ofadc
,add
,and
,cmp
,or
,sbb
,sub
,xor
instructions, when position-independent code is disabled.
Memory Operand Immediate Operand call *foo@GOT
nop call foo
call *foo@GOT
call foo nop
jmp *foo@GOT
jmp foo nop
mov foo@GOT, %reg
mov $foo, %reg
test %reg, foo@GOT
test $foo, %reg
binop foo@GOT, %reg
binop $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), %reg2
mov $foo, %reg2
test %reg1, name@GOT(%reg2)
test $foo, %reg1
binop name@GOT(%reg1), %reg2
binop $foo, %reg2