ConstantExpr Evaluation

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1

Hi,

What I'm currently working on is a translation from LLVM IR to a register transfer list format used in VPO.

If my understanding of ConstantExpr is correct, that they can be evaluated at compile-time, how can I simply have them be evaluated but have the code still remain in IR format?

For example, in:

store i32 1, i32* getelementptr [6 x i32]* @arr, i32 0, i32 0

the getelementptr ConstantExpr can be evaluated at compile-time, so is there a way to run a pass and end up with something like:

store i32 1, i32* ???

where ??? is not another ConstantExpr, but rather a ConstantInt or similar.

Thanks,
Brandon

2

Hi Brandon,

What I'm currently working on is a translation from LLVM IR to a
register transfer list format used in VPO.

If my understanding of ConstantExpr is correct, that they can be
evaluated at compile-time,

that's not so, because they may (for example) involve the address of a global
variable or other label, which is not known at compile time.

  how can I simply have them be evaluated but

have the code still remain in IR format?

For example, in:

store i32 1, i32* getelementptr [6 x i32]* @arr, i32 0, i32 0

the getelementptr ConstantExpr can be evaluated at compile-time, so is
there a way to run a pass and end up with something like:

store i32 1, i32* ???

where ??? is not another ConstantExpr, but rather a ConstantInt or similar.

It can't be, since the address @arr is not known at compile time.

Ciao, Duncan.

3

You could write a pass that makes sure that no operand of an
Instruction is a ConstantExpr, or some subset of that. There isn't
any such pass in the tree, but I recall there may have been some
discussion on llvmdev in the past. I'm not sure it saves all that
much work for your translator, though.

-Eli