[RISCV] Custom lowering of SET_ROUNDING

[jrtc27]

Would a SELECT not be more friendly to optimisations like known-bits analysis, or does that cause too much code to be emitted in the general case?

[craig.topper]

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

225

StdExtD implies StdExtF. So I think you only need to check StdExtF

904

Why do we need two code paths? Won't the other code constant fold if RMValue is a constant?

[sepavloff]

Updated patch according to reviewer's notes.

[sepavloff]

In D91242#2388630, @jrtc27 wrote:

Would a SELECT not be more friendly to optimisations like known-bits analysis, or does that cause too much code to be emitted in the general case?

Not sure if I understand you correctly, if you mean analyzing the 3 bits of rounding mode bit-by-bit, It think it does not make sense here. It is unlikely that only a part of the bits are known. Rounding mode is specified as a constant or obtained as a result of FLT_ROUNDS or as argument of a function call. Usually it is not obtained by calculations. And yes, in the general case too much code is generated, at least 8 operators, while in this implementation 5 instructions is enough.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

225

Fixed.

904

Indeed, the general case can be optimized for constant arguments.

[craig.topper]

If this gets used in a function that does floating point, what prevents the machine schedulers from reordering the FP instructions with this? Don't we need to model FRM as a register and make the FP instructions implicitly use it? You'd also need to be using the constrained FP intrinsics which aren't implemented for RISCV yet.

[sepavloff]

Repased patch

[craig.topper]

llvm/include/llvm/ADT/FloatingPointMode.h

41

This doesn't appear to be used in this patch. I don't object to adding it just wondering why it is in the RISCV patch.

llvm/test/CodeGen/RISCV/fpenv.ll

57

COMMON doesn't appear in the runline in this test.

[sepavloff]

Aligned the implementation with that of FLT_ROUNDS_

[sepavloff]

llvm/test/CodeGen/RISCV/fpenv.ll

57

I changed patterns to those generated by update_llc_test_checks.py, as the test for FLT_ROUNDS_ already uses it.

[craig.topper]

LGTM