[LLVMdev] Alias Analysis: zero terminated strings (original) (raw)

Nick Lewycky nicholas at mxc.ca
Mon Sep 12 09:31:24 PDT 2011

Carl-Philip Hänsch wrote:

Hello,

I'm developing a programming language that is optimized for strings. A first hello world program shows me that llvm needs a lot more work on zero terminated strings. In the following example, I have an auto generated hello world example optimized with -O3. The problem is, that the constant string is copied into a malloced mem area, then puts is called and then the memory is freed. There is also some leftover from the reference counters. These are found by the dead code eliminaton after the puts call. But before the puts call, the constant folded number is put into the memory and is never used. I was told that llvm assumes that a function also can read below the pointer, so dead code elimination does not work here. The second thing i would like to have there is to tell LLVM that the interesting memory ends after the zero termination. I think these two flags: dontreadbelow and dontreadabovezero should be enough to make LLVM optimze that example.

LLVM could figure out that there is no "below the pointer" by noticing that the object came from malloc. I think the missing optimization here is a heap->stack transform.

I note that in your example the exit is not post-dominated by free(). The transform could still fire by noticing that the pointer returned by malloc never escaped the function. (A more expensive check would be to see that it never escaped the function along the path that didn't call free. This is related to http://llvm.org/PR8908#c1 .)

One other thing we may want is a flag for "does not care about the pointer itself, only what the pointer points to". Currently, nothing tells LLVM that puts() doesn't check whether the pointer argument == &string_00000001, so we can't actually remove the copy. We could special-case that optimization into SimplifyLibCalls.

Nick

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