Embedded Software for SoC - Grupo de Mecatrônica EESC/USP

440 Chapter 32
block branch. All off-block branches to blocks not yet loaded must be modified
for streaming to work properly. (Clearly, the reader may infer that our
approach involves a form of binary rewriting.)
For the first case, a branch instruction which can potentially cause the
processor to execute code in another block is modified to load the block
containing the needed code as illustrated in Example 32-1. For the second
case, suppose the last instruction in the block is not a goto or a return instruction
and that the following block has not yet been loaded into memory. If left
this way, the processor might execute the next instruction. To prevent the
processor from executing invalid code, an instruction is appended by default
to load the next block; an example of this is shown at the end of Example
32-1. For the third case, no return instruction ever needs to be modified if
blocks are not allowed to be removed from memory since the instruction
after the caller instruction is always valid (even if the caller instruction is the
last instruction in the block upon initial block creation, an instruction will be
appended according to the second case above). However, if blocks are allowed
to be removed, all return instructions returning to another block must be
modified to load the other block (i.e., the caller block) if needed. For the last
case mentioned in the previous paragraph, all exception code must be streamed
before executing the block containing the corresponding exception instruction.
Therefore, exception instructions are not modified.
EXAMPLE 32-1: Consider the if-else C statement in Figure 32-3. The C
statement is compiled into corresponding PowerPC assembly instructions. The
application can be divided so that the statement is split into different blocks.
Figure 32-3 shows a possible split.
In this small example, the first block (Block 1) contains two branch instructions,
each of which could potentially jump to the second block. If the second
block (Block 2) is not in memory, this application will not work properly.
Therefore, these branch instructions must be modified. All off-block branches
are modified to invoke the appropriate loader function if the block is not in
memory. After the missing block is loaded, the intended location of the original
branch is taken. Figure 32-4 shows Block 1 and Block 2 from Figure 32-3