Fwd: Help to understand instructions

[Kalle Olavi Niemitalo]

Sergey Ivanov <icegood1980@gmail.com> writes:

>   else if (r < -halfL[i])
>   41f088:    f2 0f 10 15 90 55 02     movsd  xmm2,QWORD PTR
> [rip+0x25590]        # 444620 <typeinfo name for
> SpeciesDependedMove<GeneralPolymerChain>+0x80> <<<<<<<<<<<<<<<question
> is here

[...]

> What implicit addressation via rip means? Is it kind of protection or
> is it bug of objdump?

It just loads some constant from memory.  I suppose the compiler
did not emit a name for the constant in the debug information, so
objdump displays the nearest preceding symbol, which happens to be
typeinfo name for SpeciesDependedMove<GeneralPolymerChain>.

Somewhat reproduced like this:

#include <typeinfo>

struct Box
{
  double L[4];
  double halfL[4];
  void MinCoordDistance(double &r, const int i) const;
};

const std::type_info &dummy = typeid(Box);

void Box::MinCoordDistance(double &r, const int i) const
{
  if (r >= halfL[i])
    r -= L[i];
  else if (r < -halfL[i])
    r += L[i];
}

Compiling this with g++ (Debian 4.7.1-7) 4.7.1, options -m64 -O3 -g -S:

	.file	"xmm.c"
	.text
.Ltext0:
	.align 2
	.p2align 4,,15
	.globl	_ZNK3Box16MinCoordDistanceERdi
	.type	_ZNK3Box16MinCoordDistanceERdi, @function
_ZNK3Box16MinCoordDistanceERdi:
.LFB19:
	.file 1 "xmm.c"
	.loc 1 13 0
	.cfi_startproc
.LVL0:
	.loc 1 14 0
	movslq	%edx, %rdx
	movsd	(%rsi), %xmm0
	movsd	32(%rdi,%rdx,8), %xmm1
	ucomisd	%xmm1, %xmm0
	jae	.L10
	.loc 1 16 0
	movsd	.LC0(%rip), %xmm2       <<<<<<<<<<<<<<<question is here
	xorpd	%xmm2, %xmm1
	ucomisd	%xmm0, %xmm1
	jbe	.L1
	.loc 1 17 0
	addsd	(%rdi,%rdx,8), %xmm0
	movsd	%xmm0, (%rsi)
.L1:
	rep
	ret
	.p2align 4,,10
	.p2align 3
.L10:
	.loc 1 15 0
	subsd	(%rdi,%rdx,8), %xmm0
	movsd	%xmm0, (%rsi)
	ret
	.cfi_endproc
.LFE19:
	.size	_ZNK3Box16MinCoordDistanceERdi, .-_ZNK3Box16MinCoordDistanceERdi
	.globl	dummy
	.section	.rodata
	.align 8
	.type	dummy, @object
	.size	dummy, 8
dummy:
	.quad	_ZTI3Box
	.weak	_ZTI3Box
	.section	.rodata._ZTI3Box,"aG",@progbits,_ZTI3Box,comdat
	.align 16
	.type	_ZTI3Box, @object
	.size	_ZTI3Box, 16
_ZTI3Box:
	.quad	_ZTVN10__cxxabiv117__class_type_infoE+16
	.quad	_ZTS3Box
	.weak	_ZTS3Box
	.section	.rodata._ZTS3Box,"aG",@progbits,_ZTS3Box,comdat
	.type	_ZTS3Box, @object
	.size	_ZTS3Box, 5
_ZTS3Box:
	.string	"3Box"
	.section	.rodata.cst16,"aM",@progbits,16
	.align 16
.LC0:
	.long	0
	.long	-2147483648
	.long	0
	.long	0
	.text
... the rest omitted for brevity.

The "movsd .LC0(%rip), %xmm2" instruction loads a value from
.LC0, which is in the rodata.cst16 section and is thus a constant.
It looks like this constant is double[2] { -0.0, 0.0 } and the
function computes -halfL[i] by toggling the sign bit with xor.
The debug information in the assembly file does not refer to .LC0.