分支寄存器如何在ppc64le中工作？

Question

How does the branch register work in ppc64le?

I have the following code in armv8 -- br x19 or in armv7 -- bx r4

What is equivalent in ppc64le for it?

Would just the b r4 work or I will have to mflr r4 mr r0, r5 mtlr r4 blr

Answer 1

It sounds like what you want to do is an indirect branch. There's a couple of facilites for this on Power - the counter register, and the link register.

The link register is traditionally used for the return address when calling a function. So for example, if you have a function in asm, you might do something like:

.my_func
        // save r31 to the stack

        ...

        mflr r31   // save off link register

        ...

        bl .another_function // branch, setting the link register
        nop                  // control will return here

        ...

        mtlr r31 // restore LR
        // restore r31 from stack
        blr      // branch to LR, exiting the function

If you want to do the sort of indirect branch you're talking about in your question, you'd probably want to use the counter register. The counter register is often used for loops (hence the name) but is also very useful for indirect branches. If you're branching within a function:

mtctr r4 // r4 - address you want to go to
bctr     // unconditional branch to contents of ctr

If you want to do an indirect branch to another function, you want your branch to also set the link register:

mtctr r4
bctrl // branch to counter, setting link register

Two critical references you'll want are:

• The POWER ISA. It's available online - you might need to go through a registration portal but it should be free.
• The Power ELF ABI v2 is indispensable. It tells you all sorts of useful things like how to set up stack frames, what registers arguments are passed in, which ones are volatile/non-volatile, and more! PDF at https://members.openpowerfoundation.org/document/dl/576