This is the first part split off from D54696.
This patch adds support for the fptoui.sat and fptosi.sat intrinsics, which provide basically the same functionality as the existing fptoui and fptosi instructions, but will saturate (or return 0 for NaN) on values unrepresentable in the target type, instead of returning poison. Related mailing list discussion can be found at: https://groups.google.com/d/msg/llvm-dev/cgDFaBmCnDQ/CZAIMj4IBAAJ.
The intrinsics have overloaded source and result type and support vector operands:
i32 @llvm.fptoui.sat.i32.f32(float %f) i100 @llvm.fptoui.sat.i100.f64(double %f) <4 x i32> @llvm.fptoui.sat.v4i32.v4f16(half %f) // etc
On the SelectionDAG layer two new ISD opcodes are added, FP_TO_UINT_SAT and FP_TO_SINT_SAT. These opcodes have two operands and one result. The second operand is an integer constant specifying the scalar saturation width. The idea here is that initially the second operand and the scalar width of the result type are the same, but they may change during type legalization. For example:
i19 @llvm.fptsi.sat.i19.f32(float %f) // builds i19 fp_to_sint_sat f, 19 // type legalizes (through integer result promotion) i32 fp_to_sint_sat f, 19
I went for this approach, because saturated conversion does not compose well. There is no good way of "adjusting" a saturating conversion to i32 into one to i19 short of saturating twice. Specifying the saturation width separately allows directly saturating to the correct width.
There are two baseline expansions for the fp_to_xint_sat opcodes. If the integer bounds can be exactly represented in the float type and fminnum/fmaxnum are legal, we can expand to something like:
f = fmaxnum f, FP(MIN) f = fminnum f, FP(MAX) i = fptoxi f i = select f uo f, 0, i # unnecessary if unsigned as 0 = MIN
If the bounds cannot be exactly represented, we expand to something like this instead:
i = fptoxi f i = select f ult FP(MIN), MIN, i i = select f ogt FP(MAX), MAX, i i = select f uo f, 0, i # unnecessary if unsigned as 0 = MIN
It should be noted that this expansion assumes a non-trapping fptoxi.
Initial tests are for AArch64, x86_64 and ARM. This exercises all of the scalar and vector legalization. ARM is included to test float softening.
Original patch by @nikic.