IRTranslator

This pass translates the input LLVM-IR Function to a GMIR MachineFunction. This is typically a direct translation but does occasionally get a bit more involved. For example:

%2 = add i32 %0, %1

becomes:

%2:_(s32) = G_ADD %0:_(s32), %1:_(s32)

whereas

call i32 @puts(i8* %cast210)

is translated according to the ABI rules of the target.

Note

The currently implemented portion of the LLVM Language Reference Manual is sufficient for many compilations but it is not 100% complete. Users seeking to compile LLVM-IR containing some of the rarer features may need to implement the translation.

Target Intrinsics

There has been some (off-list) debate about whether to add target hooks for translating target intrinsics. Among those who discussed it, it was generally agreed that the IRTranslator should be able to lower target intrinsics in a customizable way but no work has happened to implement this at the time of writing.

Translating Function Calls

The IRTranslator also implements the ABI’s calling convention by lowering calls, returns, and arguments to the appropriate physical register usage and instruction sequences. This is achieved using the CallLowering implementation,

Aggregates

Caution

This has changed since it was written and is no longer accurate. It has not been refreshed in this pass of improving the documentation as I haven’t worked much in this part of the codebase and it should have attention from someone more knowledgeable about it.

Aggregates are lowered to a single scalar vreg. This differs from SelectionDAG’s multiple vregs via GetValueVTs.

TODO: As some of the bits are undef (padding), we should consider augmenting the representation with additional metadata (in effect, caching computeKnownBits information on vregs). See PR26161: [GlobalISel] Value to vreg during IR to MachineInstr translation for aggregate type

Translation of Constants

Constant operands are translated as a use of a virtual register that is defined by a G_CONSTANT or G_FCONSTANT instruction. These instructions are placed in the entry block to allow them to be subject to the continuous CSE implementation (CSEMIRBuilder). Their debug location information is removed to prevent this from confusing debuggers.

This is beneficial as it allows us to fold constants into immediate operands during InstructionSelect, while still avoiding redundant materializations for expensive non-foldable constants. However, this can lead to unnecessary spills and reloads in an -O0 pipeline, as these virtual registers can have long live ranges. This can be mitigated by running a localizer after the translator.