[llvm-dev] alloca behavior in llvm IR

[林政宗 via llvm-dev]

Hi, Tim.


I am also not quite clear about the convention of alloca. 
when the size of a variable is known at compile-time, is it always that clang will put the alloca for the variable at the start of entry block?
when will clang put alloca at other block? And when will clang put alloca at entry block?
when clang put alloca at other block, does it mean that the size of the variable of alloca is not known at compile-time?
Thanks!


Best Regards,
Jerry





At 2020-12-17 15:43:58, "Tim Northover" <t.p.northover at gmail.com> wrote:
>On Wed, 16 Dec 2020 at 13:52, 林政宗 <jackie_linzz at 126.com> wrote:
>> When there are dynamic allocas, is it always that the compiler will insert @llvm.stacksave() and @llvm.stackrestore()?
>
>Clang always emits them for variable length arrays as far as I know
>(e.g. "int arr[n]"), but C's actual "alloca" function
>(https://www.man7.org/linux/man-pages/man3/alloca.3.html) makes the
>memory available until the function returns so Clang can't use
>stackrestore anywhere there.
>
>> When there is a need to insert @llvm.stacksave() and @llvm.stackrestore(), how does the compiler decide where it should insert them?
>
>The stacksave should go just before the alloca you want to reclaim
>later, which is usually pretty easy to do.
>
>The stackrestore ideally goes just after the last instruction that
>uses the memory involved. That's difficult to determine though (and
>impossible to implement if it happens in another function). So in
>practice source languages have scoping rules that say when a variable
>is *allowed* to be accessed, and compilers put the stackrestore at the
>end of the scope.
>
>In C and C++ that's at the '}' of the block the variable is declared
>
>Cheers.
>
>Tim.
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