Branch delay
Branch delay slot in pipelining
This optimization can be performed in software at compile time by moving instructions into branch delay slots in the in-memory instruction stream, if the hardware supports this. What if one step in the line ran out of blue whatsits, and to make the blue and yellow product you need the blue whatsits. If you declare that the instruction after a branch is always executed then when a branch is taken the instruction in the decode slot also gets executed, the instruction in the fetch slot is discarded and you have one hole of time not two. In many cases, a compiler can put instructions in those slots that don't actually depend on the branch itself, but if it can't, it must fill them with NOPs, which kills the performance anyway. So what these two instructions effectively do is call a function, but set up the return address to skip the next 0x1ac bytes instructions after return. Fallacy : You can design a flawless architecture. So instead of execute, empty, empty, execute, execute you now have execute, execute, empty, execute, execute It was a simple matter to control pipeline hazards with five-stage pipelines, but a challenge for processors with longer pipelines that issue multiple instructions per clock cycle. Each cycle where a stall is inserted is considered one branch delay slot. As you would expect from the design of a CPU pipeline, the CPU basically executes the branch and the delay instruction in order, as they are stored in the instruction stream, and it only delays the write to PC, i. What if the delay instruction writes r5 and the branch jumps to r5? Various systems have different ways of improving the accuracy of the guess. No instruction executes more than once. On the aforementioned Motorola M88K, this behaviour is documented, and GCC even makes use of it: 7d ad 00 08 cmp r13,r13,0x08 ; compare d4 6d 00 05 bb0. ARM does not have a delay slot, but it gives the illusion of a pipeline as well, by declaring that the program counter is two instructions ahead.No instruction executes more than once. Another stupid branch delay slot trick is editing the return address as part of the jump. Each cycle where a stall is inserted is considered one branch delay slot.
Delayed branch in pipelining ppt
The idea of the branch shadow or delay slot is to recover one of those clocks. Otherwise, you can fill it with a NOP. Without them, the pipeline needs to stall whenever a conditional branch is taken, because the instruction fetch mechanism can't know which instruction should be executed next after the branch instruction until the computations on which it depends are completed. This optimization can be performed in software at compile time by moving instructions into branch delay slots in the in-memory instruction stream, if the hardware supports this. Fallacy : You can design a flawless architecture. If you declare that the instruction after a branch is always executed then when a branch is taken the instruction in the decode slot also gets executed, the instruction in the fetch slot is discarded and you have one hole of time not two. Over time those technologies are likely to change, and decisions that may have been correct at the time they were made look like mistakes. A simple design would insert stalls into the pipeline after a branch instruction until the new branch target address is computed and loaded into the program counter. A more sophisticated design would execute program instructions which are not dependent on the result of the branch instruction. Bonus chatter: Another extra sneaky trick is reusing the return address. Load delay slots are very uncommon because load delays are highly unpredictable on modern hardware. This approach keeps the hardware simple, but puts a burden on the compiler technology. How deep the pipes really are is often not shared with the public. See footnote 1.Visualize an assembly line, each step in the line has a task. What if one step in the line ran out of blue whatsits, and to make the blue and yellow product you need the blue whatsits. Indeed, in terms of software, delayed branch only has drawbacks as it makes programs more difficult to read and less efficient as the slot is frequently filled by nops.
Macro instruction expanded into multiple instructions in a branch delay slot
Registers R0 through R9 are cleared to zero in order by number the register cleared after R6 is R7, not R9. Branch prediction is a more hardware-oriented approach, in which the instruction fetcher simply "guesses" which way the branch will go, executes instructions down that path, and if it later turns out to have guessed wrong, the results of those instructions are thrown away. With present branch predictors, misprediction is by far lower than the number of branches with a useless nop delay slot and is accordingly more efficient, even on a 6 cycles computer like nios-f. The MIPS designers codified existing practice and retroactively declared that if the register operand in the JR instruction is ra, then it predicts as a subroutine return; otherwise it predicts as a computed jump. Software compatibility requirements dictate that an architecture may not change the number of delay slots from one generation to the next. Load delays were seen on very early RISC processor designs. The branch delay slot is a side effect of pipelined architectures due to the branch hazard , i. It was a simple matter to control pipeline hazards with five-stage pipelines, but a challenge for processors with longer pipelines that issue multiple instructions per clock cycle. Over time those technologies are likely to change, and decisions that may have been correct at the time they were made look like mistakes.That is like what happens with a branch, somewhere deep in the assembly line, something causes the line to have to change, dump the line. And you cant get new blue whatsits for another week because someone screwed up.
We also were able to remove the duplicate OR v0, zero, zero instruction that had been hoisted into the branch delay slot of the unconditional branch.
Delayed branch simply means that some number of instructions that appear after the branch in the instruction stream will be executed regardless of which way the branch ultimately goes.
A more sophisticated design would execute program instructions which are not dependent on the result of the branch instruction.
So in practice, you can put the instruction that would be before the branch right after the branch, if this instruction is independent of the branch instruction, i.
So what these two instructions effectively do is call a function, but set up the return address to skip the next 0x1ac bytes instructions after return.
