Assembler and Dissassembler

LLVM provides two different representation after generating the hardware instructions. The .s file (assembly representation) and .o file ( the object file). They are just two different "IR". If we want to generate the .o file from .s file, we need to call the assembler (usually /usr/bin/as). If we want to do the opposite, we can use objdump -d (meaning disassemble)

The assembler and disassembler together can be used to modify the assembly and do some testing.

e.g. :

clang a.c -S -O2 -o a.s

do some modifications to the a.s

clang a.s -o a.out (here clang implicitly invoke the assembler to generate exe)

You can also do (equivalent to the above):

assemble:

/usr/bin/as  a.s -o a.o

clang a.o -o a.out 

To look at the assembly of .o or exe

disassemble:

objdump modified.o-d > modified.out.disassembly

objdump modified.out -d > modified.out.disassembly

How to access global variables and TOC

int a = 2;
int b = 3;

int foo(void) {
  printf("a+b=%d",a + b);
  return 0;
}



foo:                                    # @foo
.Lfunc_begin0:
.Lfunc_gep0:
        addis r2, r12, .TOC.-.Lfunc_gep0@ha
        addi r2, r2, .TOC.-.Lfunc_gep0@l
.Lfunc_lep0:
        .localentry     foo, .Lfunc_lep0-.Lfunc_gep0
# BB#0:                                 # %entry
        mflr r0
        std r0, 16(r1) # save the link register to 16(r1), i.e stackFrame + 16`
        stdu r1, -96(r1)  # prolog
        addis r3, r2, .LC0@toc@ha
        addis r4, r2, .LC1@toc@ha
        addis r12, r2, .L.str@toc@ha
        ld r3, .LC0@toc@l(r3)
        ld r4, .LC1@toc@l(r4)
        lwz r3, 0(r3)
        lwz r4, 0(r4)
        add r3, r4, r3
        extsw r4, r3
        addi r3, r12, .L.str@toc@l
        bl printf
        nop  # may insert tls depending on the where is the callee definition.
        li r3, 0
        addi r1, r1, 96  # the reverse of the 3rd instr
        ld r0, 16(r1) # load the old link reigster address
        mtlr r0  # restore the link register value
        blr
        .long   0
        .quad   0
.Lfunc_end0:
        .size   foo, .Lfunc_end0-.Lfunc_begin0

4.3.2 Data Cache Instructions (DCBT/dcbt)

\brief: DCBT is one of the Data Cache instruction used on PowerPC.

The Data Cache instructions control various aspects of
the data cache.
TH field in the dcbt and dcbtst instructions
Described below are the TH field values for the dcbt
and dcbtst instructions. For all TH field values which
are not listed, the hint provided by the instruction is
undefined.
TH=0b00000
If TH=0b00000, the dcbt/dcbtst instruction provides a
hint that the program will probably soon access the
block containing the byte addressed by EA.
TH=0b01000 - 0b01111
The dcbt/dcbtst instructions provide hints regarding a
sequence of accesses to data elements, or indicate the
expected use thereof. Such a sequence is called a
“data stream”, and a dcbt/dcbtst instruction in which
TH is set to one of these values is said to be a “data
stream variant” of dcbt/dcbtst. In the remainder of this
section, “data stream” may be abbreviated to “stream”.
A data stream to which a program may perform Load
accesses is said to be a “load data stream”, and is
described using the data stream variants of the dcbt
instruction. A data stream to which a program may perform
Store accesses is said to be a “store data stream”,
and is described using the data stream variants of the
dcbtst instruction.
When, and how often, effective addresses for a data
stream are translated is implementation-dependent.
Each data element is associated with a unit of storage,