[llvm-dev] tool options to generate spill code

[Priyanka Panigrahi via llvm-dev]

Thanks for the clarification. Why does mem2reg remove the dead stores?

Regards,
Priyanka

On Wed, Feb 12, 2020 at 9:25 PM Aaron Smith <aaron.lee.smith at gmail.com>
wrote:

> 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
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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