[llvm-dev] Need help: How to turn off the constant folding optimization in llvm

[Cranmer, Joshua via llvm-dev]

The way that the front-end lowers code to IR causes this sort of constant folding to happen even before any LLVM IR is generated. Essentially, when you do the AST traversal, you’re going to essentially see the following code get run:

IRBuilder<> Builder;
Value *LHS = Builder.getInt32(2);
Value *RHS = Builder.getInt32(4);
// LHS and RHS are ConstantInt values because they’re constant expressions.
Value *Res = Builder.CreateMul(LHS, RHS); // Because LHS and RHS are constant values, the IRBuilder folds this to a constant expression.

This constant folding cannot be turned off. (I’m also assuming there’s no other constant folding going on at the Clang AST level).

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Md. Syadus Sefat via llvm-dev
Sent: Sunday, January 20, 2019 19:30
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] Need help: How to turn off the constant folding optimization in llvm

Hi, I am new to clang and llvm. I'm trying to generate an unoptimized version of bit code from a c source code. I found that the generated bit code is having the constant folding optimization which I don't want.
I'm using this command: clang -O0 -Xclang -disable-O0-optnone test1.c -S -emit-llvm -o test1.ll

The test1.c file has the following code:
int test(){
int y;
y = 2 * 4;
return y;
}

The content of the test1.ll file:
[image.png]
Instead of generating an instruction for multiplying 2 and 4, it is directly storing the value 8 by doing the constant folding operation.
store i32 8, i32* %1, align 4

It would be really nice if someone kindly let me know what I am missing and how should I turn off the constant folding optimization.

Thank you.
Regards,
SS









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