CODEWITHSUNDEEP
c++gccassemblyx86inline-assembly
Yuan Zhu
Yuan Zhu

Reputation: 59

How to force GCC to use jmp instruction instead of ret?

I was now using a stackful co-routines for network programming. But I was punished by the invalidation of return stack buffer (see http://www.agner.org/optimize/microarchitecture.pdf p.36), during the context switch (because we manually change the SP register)

I found out that the jmp instruction is better than ret after assembly language test. However, I have some more functions that indirectly call the context switch function that was written in C++ language (compiled by GCC). How can we force these function return using jmp instead of ret in the GCC assembly result?

Some common but not perfect methods:

  1. using inline assembly and manually set SP register to __builtin_frame_address+2*sizeof(void*) and jmp to the return address, before ret?

This is an unsafe solution. In C++, local variables or right values are destructed before ret instruction. We will omit these instruction if we jmp. What's worse, even if we are in C, callee-saved registers need to be restored before ret instruction and we will also omit these instruction, too.

So what can we do to force GCC use jmp instead of ret to avoid the problems listing above?

Upvotes: 3

Views: 1405

Answers (1)

Timothy Baldwin
Timothy Baldwin

Reputation: 3675

Use an assembler macro:

.macro ret
       pop %ecx
       jmp *%ecx
.endm

Put that in inline assembler at the top of the file or elsewhere.

Upvotes: 3

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