[llvm-dev] tool options to generate spill code

[Aaron Smith via llvm-dev]

If you use -O0 to generate LLVM IR you must pass -disable-O0-optnone or it
cannot be optimized. If you look at the LLVM IR for #2 and #3 it will say
"optnone" [1] in the attributes which will prevent any optimization like
mem2reg [2] from running.

[1] https://clang.llvm.org/docs/AttributeReference.html#optnone
[2] http://llvm.org/docs/Passes.html#mem2reg-promote-memory-to-register


On Wed, Feb 12, 2020 at 2:36 AM Priyanka Panigrahi via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> Hello,
>
> For the following test case, reg.c
>
> #include <stdio.h>
>
> int getinput()
> {
> static int u=10;
> return u++;
> }
>
> int main()
> {
>
> int a,b,c,d,e,f,g;
>
>         a=getinput();
>         b=getinput();
>         c=getinput();
>         d=getinput();
>         e=getinput();
>         f=getinput();
>         g=getinput();
>
> printf("%d %d %d %d %d %d %d\n",a,b,c,d,e,f,g);
> a=b=c=d=e=f=g=0;
> return 0;
> }
>
> *1. $clang reg.c -Xclang -disable-O0-optnone -S -emit-llvm -o reg.ll &&
> opt -mem2reg -S reg.ll -o reg.ll && llc --regalloc=greedy reg.ll -o reg.s*
>
> *2. $clang reg.c -Xclang -disable-llvm-passes -S -emit-llvm -o reg.ll &&
> opt -mem2reg -S reg.ll -o reg.ll && llc --regalloc=greedy reg.ll -o reg.s*
>
> *3. $clang reg.c -S -emit-llvm -o reg.ll && opt -mem2reg -S reg.ll -o
> reg.ll && llc --regalloc=greedy reg.ll -o reg.s*
>
> Only 1. gives the spill code but it deletes the dead stores., 2 and 3 give
> only allocas. Why so??
>
> How -mem2reg works and what it depends on?
>
> How to generate the spill code with dead stores?
>
> Thank you.
>
> Regards,
> Priyanka
> _______________________________________________
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> llvm-dev at lists.llvm.org
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>
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