Hello, I would just like to ask if it's possible to explicitly free allocas. This is because I need to call functions that take structs of different sizes as input, (possibly inside of loops) and I would rather avoid a stack overflow. If this is not possible, an alternate solution would be for me to allocate an array of bytes larger than all the struct types I may be using, and cast that pointer to the right type each type. In that case, I would like to know how I can find out what the size of a struct is. I've heard of people talking about using "the right target data", but I have no idea how to actually do that, and I find the doxygen info doesn't make it any clearer. Thank you for your help, - Max -- View this message in context: http://www.nabble.com/Explicitly-Freeing-Allocas-tp24093351p24093351.html Sent from the LLVM - Dev mailing list archive at Nabble.com.
Nyx wrote:> Hello, > > I would just like to ask if it's possible to explicitly free allocas. This > is because I need to call functions that take structs of different sizes as > input, (possibly inside of loops) and I would rather avoid a stack overflow. >No, it's not legal to free memory returned by alloca. Such memory is allocated on the stack; the code generator usually converts this to an adjustment of the stack pointer on function entry whenever possible.> If this is not possible, an alternate solution would be for me to allocate > an array of bytes larger than all the struct types I may be using, and cast > that pointer to the right type each type. In that case, I would like to know > how I can find out what the size of a struct is. I've heard of people > talking about using "the right target data", but I have no idea how to > actually do that, and I find the doxygen info doesn't make it any clearer. >This sounds like your best option. The TargetData pass is a pass that informs other passes of machine specific information and can, for example, give you the number of bytes used to store a type. Read the document on Writing an LLVM Pass (http://llvm.org/docs/WritingAnLLVMPass.html) to learn how to have your pass declare the TargetData pass as a prerequisite and to get a pointer to it when your runOnFunction/runOnModule/runOnBasicBlock/runOn<whatever> method is called. Then use the doxygen documentation to find the correct method of the TargetData pass (http://llvm.org/doxygen/classllvm_1_1TargetData.html) to use to get the size of the type that you allocated via alloca. -- John T.> Thank you for your help, > > - Max >
Nyx wrote:> Hello, > > I would just like to ask if it's possible to explicitly free allocas. This > is because I need to call functions that take structs of different sizes as > input, (possibly inside of loops) and I would rather avoid a stack overflow.You can't explicitly free a specific alloca, but you can use the llvm.stacksave and llvm.stackrestore intrinsics to free all allocas performed after calling the stacksave intrinsic. http://llvm.org/docs/LangRef.html#int_stacksave
That sounds rather cumbersome, is there no simpler way to get the actual size of a struct? John Criswell wrote:> > Nyx wrote: >> Hello, >> >> I would just like to ask if it's possible to explicitly free allocas. >> This >> is because I need to call functions that take structs of different sizes >> as >> input, (possibly inside of loops) and I would rather avoid a stack >> overflow. >> > No, it's not legal to free memory returned by alloca. Such memory is > allocated on the stack; the code generator usually converts this to an > adjustment of the stack pointer on function entry whenever possible. >> If this is not possible, an alternate solution would be for me to >> allocate >> an array of bytes larger than all the struct types I may be using, and >> cast >> that pointer to the right type each type. In that case, I would like to >> know >> how I can find out what the size of a struct is. I've heard of people >> talking about using "the right target data", but I have no idea how to >> actually do that, and I find the doxygen info doesn't make it any >> clearer. >> > This sounds like your best option. The TargetData pass is a pass that > informs other passes of machine specific information and can, for > example, give you the number of bytes used to store a type. > > Read the document on Writing an LLVM Pass > (http://llvm.org/docs/WritingAnLLVMPass.html) to learn how to have your > pass declare the TargetData pass as a prerequisite and to get a pointer > to it when your > runOnFunction/runOnModule/runOnBasicBlock/runOn<whatever> method is > called. Then use the doxygen documentation to find the correct method > of the TargetData pass > (http://llvm.org/doxygen/classllvm_1_1TargetData.html) to use to get the > size of the type that you allocated via alloca. > > -- John T. > >> Thank you for your help, >> >> - Max >> > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > >-- View this message in context: http://www.nabble.com/Explicitly-Freeing-Allocas-tp24093351p24094996.html Sent from the LLVM - Dev mailing list archive at Nabble.com.
What kind of overhead does that have? Does it only restore the stack pointer to what it was (what I want), or does it do more than that, and try to restore values that were on the stack prior to save, etc? Frits van Bommel wrote:> > Nyx wrote: >> Hello, >> >> I would just like to ask if it's possible to explicitly free allocas. >> This >> is because I need to call functions that take structs of different sizes >> as >> input, (possibly inside of loops) and I would rather avoid a stack >> overflow. > > You can't explicitly free a specific alloca, but you can use the > llvm.stacksave > and llvm.stackrestore intrinsics to free all allocas performed after > calling the > stacksave intrinsic. > > http://llvm.org/docs/LangRef.html#int_stacksave > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > >-- View this message in context: http://www.nabble.com/Explicitly-Freeing-Allocas-tp24093351p24095072.html Sent from the LLVM - Dev mailing list archive at Nabble.com.
Hi,> If this is not possible, an alternate solution would be for me to allocate > an array of bytes larger than all the struct types I may be using, and cast > that pointer to the right type each type. In that case, I would like to know > how I can find out what the size of a struct is. I've heard of people > talking about using "the right target data", but I have no idea how to > actually do that, and I find the doxygen info doesn't make it any clearer.you can get the size in a target independent way using ConstantExpr::getSizeOf. You can also get the alignment using ConstantExpr::getAlignOf, but this is less useful. Ciao, Duncan.
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