llvm-project

222f4e49 - [mlir] Add FP software implementation lowering pass: `arith-to-apfloat` (#166618)

[mlir] Add FP software implementation lowering pass: `arith-to-apfloat` (#166618) This commit adds a new pass that lowers floating-point `arith` operations to calls into the execution engine runtime library. Currently supported operations: `addf`, `subf`, `mulf`, `divf`, `remf`. All floating-point types that have an APFloat semantics are supported. This includes low-precision floating-point types such as `f4E2M1FN` that cannot execute natively on CPUs. This commit also improves the `vector.print` lowering pattern to call into the runtime library for floating-point types that are not supported by LLVM. This is necessary to write a meaningful integration test. The way it works is ```mlir func.func @full_example() { %a = arith.constant 1.4 : f8E4M3FN %b = func.call @foo() : () -> (f8E4M3FN) %c = arith.addf %a, %b : f8E4M3FN vector.print %c : f8E4M3FN return } ``` gets transformed to ```mlir func.func private @__mlir_apfloat_add(i32, i64, i64) -> i6 func.func @full_example() { %cst = arith.constant 1.375000e+00 : f8E4M3FN %0 = call @foo() : () -> f8E4M3FN // bitcast operand A to integer of equal width %1 = arith.bitcast %cst : f8E4M3FN to i8 // zext A to i64 %2 = arith.extui %1 : i8 to i64 // same for operand B %3 = arith.bitcast %0 : f8E4M3FN to i8 %4 = arith.extui %3 : i8 to i64 // get the llvm::fltSemantics(f8E4M3FN) as an enum %c10_i32 = arith.constant 10 : i32 // call the impl against APFloat in mlir_apfloat_wrappers %5 = call @__mlir_apfloat_add(%c10_i32, %2, %4) : (i32, i64, i64) -> i64 // "cast" back to the original fp type %6 = arith.trunci %5 : i64 to i8 %7 = arith.bitcast %6 : i8 to f8E4M3FN vector.print %7 : f8E4M3FN } ``` Note, `llvm::fltSemantics(f8E4M3FN)` is emitted by the pattern each time an `arith` op is transformed, thereby making the call to `__mlir_apfloat_add` correct (i.e., no name mangling on type necessary). RFC: https://discourse.llvm.org/t/rfc-software-implementation-for-unsupported-fp-types-in-convert-arith-to-llvm/88785 --------- Co-authored-by: Matthias Springer <me@m-sp.org>