[LLVMdev] llvm.lifetime.start; what does it do exactly? (original) (raw)
Duncan Sands baldrick at free.fr
Fri Jul 13 09:40:20 PDT 2012
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Hi Ryan,
On 13/07/12 17:58, ryan baird wrote:
Esentially, I'm working on a translator someone started building for llvm2.9 that translates an optimized .lln file to another intermediate language, and I'm porting it to 3.1.
There is a new intrinsic that pops up in our test cases' lln files that never used to, llvm.lifetime.start. I looked up the description: "The 'llvm.lifetime.start' intrinsic specifies the start of a memory object's lifetime." However, it's still not clear to me what exactly i should do; should this actually be translated to anything (like a call to malloc or something), or is it just used to keep information about the memory object?
you can safely ignore it. The basic use case is this: function A calls function B, and the optimizers inline the call. Local variables of (the inlined) B now get a declaration at the start of A, but they are only actually used in the inlined B code, not before nor after. Adding a lifetime.start just before the inlined code, and a lifetime.end just after, explains this to the optimizers and code generators, which makes it easier for them to exploit this information.
Ciao, Duncan.
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