Tips and tricks

First of all, read the troubleshooting page. It explains how to deal with most decompilation problems. Below is a mix of other useful information that did not fit into any other page:

Volatile memory

Sometimes the decompiler can be overly aggressive and optimize references to volatile memory completely away. A typical situation like the following:

                 device_ready    DCD ? ; VOLATILE!

                                 MOV     R0, =device_ready
                                 LDR     R1, [R0]
                 LOOP:
                                 LDR     R2, [R0]
                                 SUB     R2, R1
                                 BEQ     LOOP

can be decompiled into

                 while ( 1 )
                   ;

because the decompiler assumes that a variable cannot change its value by itself and it can prove that r0 continues to point to the same location during the loop.

To prevent such optimization, we need to mark the variable as volatile. Currently the decompiler considers memory to be volatile if it belongs to a segment with one of the following names: IO, IOPORTS, PORTS, VOLATILE. The character case is not important.

Constant memory

Sometimes the decompiler does not optimize the code enough because it assumes that variables may change their values. For example, the following code:

                   LDR     R1, =off_45934
                   MOV     R2, #0
                   ADD     R3, SP, #0x14+var_C
                   LDR     R1, [R1]
                   LDR     R1, [R1]        ; int
                   BL      _IOServiceOpen

can be decompiled into

                 IOServiceOpen(r0_1, *off_45934, 0)

but this code is much better:

                 IOServiceOpen(r0_1, mach_task_self, 0)

because

                 off_45934 DCD _mach_task_self

is a pointer that resides in constant memory and will never change its value.

The decompiler considers memory to be constant if one of the following conditions hold:

the segment has access permissions defined but the write permission is not in the list (to change the segment permissions use the "Edit, Segments, Edit Segment" menu item or the set_segm_attr built-in function)
the segment type is CODE
the segment name is one of the following (the list may change in the future): .text, .rdata, .got, .got.plt, .rodata, __text, __const, __const_coal, __cstring, __cfstring, __literal4, __literal8, __pointers, __nl_symbol_ptr, __la_symbol_ptr, __objc_catlist, __objc_classlist, __objc_classname, __objc_classrefs, __objc_const, __objc_data, __objc_imageinfo, __objc_ivar, __objc_methname, __objc_methtype, __objc_protolist, __objc_protorefs, __objc_selrefs, __objc_superrefs, __message_refs, __cls_refs, __inst_meth, __cat_inst_meth, __cat_cls_meth, __OBJC_RO
The decompiler tries to completely get rid of references to the following segments and replace them by constants: .got, .got.plt, __pointers, __nl_symbol_ptr, __la_symbol_ptr, __objc_ivar, __message_refs, __cls_refs

It is possible to override the constness of an individual item by specifying its type with the volatile or const modifiers.

CONTAINING_RECORD macro

The decompiler knows about the CONTAINING_RECORD macro and tries to use it in the output. However, in most cases it is impossible to create this macro automatically, because the information about the containing record is not available. The decompiler uses three sources of information to determine if CONTAINING_RECORD should be used:

If there is an assignment like this:
```
            v1 = (structype *)((char *)v2 - num);
        
```
it can be converted into
```
            v1 = CONTAINING_RECORD(v2, structype, fieldname);
        
```
by simply confirming the types of v1 and v2. NOTE: the variables types must be specified explicitly. Even if the types are displayed as correct, the user should press Y followed by Enter to confirm the variable type.
Struct offsets applied to numbers in the disassembly listing are used as a hint to create CONTAINING_RECORD. For example, applying structure offset to 0x41C in
```
            sub     eax, 41Ch
        
```
will have the same effect as in the previous point. Please note that it makes sense to confirm the variable types as explained earlier.
Struct offsets applied to numbers in the decompiler output. For example, applying _DEVICE_INFO structure offset to -131 in the following code:
```
            deviceInfo = (_DEVICE_INFO *)((char *)&thisEntry[-131] - 4);
        
```
will convert it to:
```
            deviceInfo = CONTAINING_RECORD(thisEntry, _DEVICE_INFO, ListEntry);
        
```
Please note that it makes sense to confirm the variable types as explained earlier.

In most cases the CONTAING_RECORD macro can be replaced by a shorter and nicer expression if a shifted pointer used. In this case it is enough to declare the pointer as a shifted pointer and the decompiler will transform all expressions where it is used.

Indirect calls

Since the arguments of indirect calls are collected before defining variables, specifying the type of the variable that holds the function pointer may not be enough. The user have to specify the function type using other methods in this case. The following methods exist (in the order of preference):

For indirect calls of this form:
```
            call ds:funcptr
        
```
If funcptr is initialized statically and points to a valid function, just ensure a correct function prototype. The decompiler will use it.
For indirect calls of this form:
```
            call [reg+offset]
        
```
If reg points to a structure with a member that is a function pointer, just convert the operand into a structure offset (hotkey T):
```
            call [reg+mystruct.funcptr]
        
```
and ensure that the type of mystruct::funcptr is a pointer to a function of the desired type.
Specify the type of the called function using Edit, Operand type, Set operand type. If the first two methods cannot be applied, this is the recommended method. The operand type has the highest priority, it is always used if present.
If the address of the called function is known, use Edit, Plugins, Change the callee address (hotkey Alt-F11). The decompiler will use the type of the specified callee. This method is available only for x86. For other processors adding a code cross reference from the call instruction to the callee will help.

Last updated 3 months ago