Fwd: Help to understand instructions

Fwd: Help to understand instructions

[Sergey Ivanov]

Have compiled program with -g O3 together.
Now learn it output via objdump (with code mixed format) and see
(sorry for rather long one, just environment for question):


..........
inline void Box::MinCoordDistance(double &r, const int i) const
{
  if (r >= halfL[i])
  41f06d:    f2 0f 10 48 38           movsd  xmm1,QWORD PTR [rax+0x38]
*/
inline double Box::distanceSq(const Vec r1, const Vec r2, Vec dr) const
{
  for (int i = 0; i < 3; i++)
  {
    dr[i] = r1[i] - r2[i];
  41f072:    f2 41 0f 5c 40 10        subsd  xmm0,QWORD PTR [r8+0x10]
  return distanceSq(r1, r2, dr);

}

inline void Box::MinCoordDistance(double &r, const int i) const
{
  if (r >= halfL[i])
  41f078:    66 0f 2e c1              ucomisd xmm0,xmm1
*/
inline double Box::distanceSq(const Vec r1, const Vec r2, Vec dr) const
{
  for (int i = 0; i < 3; i++)
  {
    dr[i] = r1[i] - r2[i];
  41f07c:    f2 0f 11 44 24 18        movsd  QWORD PTR [rsp+0x18],xmm0
  return distanceSq(r1, r2, dr);

}

inline void Box::MinCoordDistance(double &r, const int i) const
{
  if (r >= halfL[i])
  41f082:    0f 83 50 01 00 00        jae    41f1d8
<CbDoubleBridgeBase<EnsembleNVT,
Homopolymer>::scanBridgingForwardOne(int)+0x258>

    r -= L[i];
  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
  41f08f:    00
  41f090:    66 0f 57 ca              xorpd  xmm1,xmm2
  41f094:    66 0f 2e c8              ucomisd xmm1,xmm0
  41f098:    76 0b                    jbe    41f0a5
<CbDoubleBridgeBase<EnsembleNVT,
Homopolymer>::scanBridgingForwardOne(int)+0x125>
    r += L[i];
  41f09a:    f2 0f 58 40 20           addsd  xmm0,QWORD PTR [rax+0x20]
  41f09f:    f2 0f 11 44 24 18        movsd  QWORD PTR [rsp+0x18],xmm0
*/
inline double Box::distanceSq(const Vec r1, const Vec r2, Vec dr) const
{
  for (int i = 0; i < 3; i++)
  {
    dr[i] = r1[i] - r2[i];
  41f0a5:    f2 0f 10 47 18           movsd  xmm0,QWORD PTR [rdi+0x18]
  return distanceSq(r1, r2, dr);

}

inline void Box::MinCoordDistance(double &r, const int i) const
{
  if (r >= halfL[i])
  41f0aa:    f2 0f 10 48 40           movsd  xmm1,QWORD PTR [rax+0x40]
*/
inline double Box::distanceSq(const Vec r1, const Vec r2, Vec dr) const
{
  for (int i = 0; i < 3; i++)
  {
    dr[i] = r1[i] - r2[i];
  41f0af:    f2 41 0f 5c 40 18        subsd  xmm0,QWORD PTR [r8+0x18]
  return distanceSq(r1, r2, dr);
}
.................

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


--
Kind regards,
Sergey Ivanov

Re: 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.