[llvm-dev] How to insert instructions before each function calls?

Dean Michael Berris via llvm-dev llvm-dev at lists.llvm.org
Sun Sep 4 23:05:13 PDT 2016

> On 5 Sep 2016, at 11:49, SHUCAI YAO <yaos4 at mcmaster.ca> wrote:
> For the function call lowering, do you mean lowercall function? I did insert the instruction before the stack adjustments, but the inserted code appears in the prologue of the function, other than before the function call. Maybe I did something wrong with the iterator. 
> So you mean I should insert a pseudo instruction in the machinefunction pass, then replace it when function call lowering?  (Like segemented stack implementation?)

Yes, inserting pseudo instructions in the MachineFunctionPass -- you might want to have a look at PATCHABLE_RET and how we handle this in XRay. Essentially the idea (which I saw Sanjoy Das do first) is to wrap the actual instruction (in this case, CALL or LEA, or something specific in the platform you're targeting) in a pseudo instruction that just lowers to the correct sequence. This gives you complete control of the actual assembly of the instructions that you're replacing.

-- Dean

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