leonardchan marked an inline comment as done. leonardchan added inline comments.
================ Comment at: lib/Sema/SemaExpr.cpp:1248 + bool RHSFixed = RHSType->isFixedPointType(); + + if (LHSFixed && RHSFixed) { ---------------- ebevhan wrote: > leonardchan wrote: > > leonardchan wrote: > > > ebevhan wrote: > > > > ebevhan wrote: > > > > > leonardchan wrote: > > > > > > leonardchan wrote: > > > > > > > ebevhan wrote: > > > > > > > > ebevhan wrote: > > > > > > > > > leonardchan wrote: > > > > > > > > > > ebevhan wrote: > > > > > > > > > > > leonardchan wrote: > > > > > > > > > > > > ebevhan wrote: > > > > > > > > > > > > > I don't see how these semantics work properly. The > > > > > > > > > > > > > specification requires that operations be done in the > > > > > > > > > > > > > full precision of both types. You cannot convert the > > > > > > > > > > > > > types before performing the operation like this, > > > > > > > > > > > > > since the operation will not be done in full > > > > > > > > > > > > > precision in that case. > > > > > > > > > > > > > > > > > > > > > > > > > > The operator semantics of Embedded-C require the > > > > > > > > > > > > > operand types of binary operators to be different. > > > > > > > > > > > > > It's only when you've performed the operation that > > > > > > > > > > > > > you are allowed to convert the result to the > > > > > > > > > > > > > resulting type. > > > > > > > > > > > > Initially the idea was to convert both sides to fixed > > > > > > > > > > > > point types, then perform standard binary operations > > > > > > > > > > > > between the fixed point types. > > > > > > > > > > > > > > > > > > > > > > > > For the example, a `fract * int` would have the int > > > > > > > > > > > > converted to a fixed point type by left shifting it by > > > > > > > > > > > > the scale of the fract, multiplying, then right > > > > > > > > > > > > shifting by the scale again to get the resulting fract. > > > > > > > > > > > > The only unhandled thing is overflow, but the precision > > > > > > > > > > > > of the fract remains the same. The operands would also > > > > > > > > > > > > be casted up beforehand so there was enough space to > > > > > > > > > > > > store the result, which was casted down back to the > > > > > > > > > > > > original fract after performing the right shift by the > > > > > > > > > > > > scale. > > > > > > > > > > > > > > > > > > > > > > > > Operations between fixed point types would follow a > > > > > > > > > > > > similar process of casting both operands to the higher > > > > > > > > > > > > rank fixed point type, and depending on the operation, > > > > > > > > > > > > more underlying shifting and casting would be done to > > > > > > > > > > > > retain full precision of the higher ranked type. > > > > > > > > > > > > > > > > > > > > > > > > Though I will admit that I did not realize until now > > > > > > > > > > > > that multiplying a fixed point type by an integer does > > > > > > > > > > > > not require shifting the integer. > > > > > > > > > > > I see how you've reasoned; this is how C normally works. > > > > > > > > > > > The `fract` is of higher rank than `int` and therefore is > > > > > > > > > > > the 'common type' of the operation. However, even though > > > > > > > > > > > it is higher rank there is no guarantee that you can > > > > > > > > > > > perform the operation without overflowing. And overflow > > > > > > > > > > > matters here; the spec says that it must be done in the > > > > > > > > > > > full precision (integral + fractional) of both types. > > > > > > > > > > > > > > > > > > > > > > > The only unhandled thing is overflow, but the precision > > > > > > > > > > > > of the fract remains the same. The operands would also > > > > > > > > > > > > be casted up beforehand so there was enough space to > > > > > > > > > > > > store the result, which was casted down back to the > > > > > > > > > > > > original fract after performing the right shift by the > > > > > > > > > > > > scale. > > > > > > > > > > > > > > > > > > > > > > The precision remains the same (and while it doesn't have > > > > > > > > > > > to be the same to perform an operation, it makes the > > > > > > > > > > > implementation more regular; things like addition and > > > > > > > > > > > subtraction 'just work'), but you cannot perform a > > > > > > > > > > > conversion to `fract` *before* the operation itself, > > > > > > > > > > > since if you do, there's nothing to 'cast up'. Casting up > > > > > > > > > > > is needed for things like `fract * fract` to prevent > > > > > > > > > > > overflow, but for `fract * int` you need to cast to a > > > > > > > > > > > type that can fit both all values of the int and all > > > > > > > > > > > values of the fract, and *then* you can cast up before > > > > > > > > > > > doing the multiplication. > > > > > > > > > > > > > > > > > > > > > > > Operations between fixed point types would follow a > > > > > > > > > > > > similar process of casting both operands to the higher > > > > > > > > > > > > rank fixed point type, and depending on the operation, > > > > > > > > > > > > more underlying shifting and casting would be done to > > > > > > > > > > > > retain full precision of the higher ranked type. > > > > > > > > > > > > > > > > > > > > > > This might work, but I feel there could be edge cases. > > > > > > > > > > > The E-C fixed-point ranks are very odd as they don't > > > > > > > > > > > reflect reality; `short _Accum` cannot be considered > > > > > > > > > > > strictly 'above' `long _Fract`, but the former has a > > > > > > > > > > > higher rank than the latter. Depending on how the types > > > > > > > > > > > are specified for a target, implicit casts between > > > > > > > > > > > fixed-point types might inadvertantly discard bits, even > > > > > > > > > > > though the spec says that operations must be done in full > > > > > > > > > > > precision. > > > > > > > > > > I see, so just to confirm, something like a `fract * int` > > > > > > > > > > would not result in any implicit casting between either > > > > > > > > > > operand, but any special arithmetic, like intermediate > > > > > > > > > > storage types or saturation handling, would be handled by > > > > > > > > > > the underlying IR? > > > > > > > > > > > > > > > > > > > > So should really no conversions/implicit type casting > > > > > > > > > > should be performed here and instead all handling of > > > > > > > > > > arithmetic operations should happen somewhere during the > > > > > > > > > > codegen stage? > > > > > > > > > > > > > > > > > > > > I see, so just to confirm, something like a fract * int > > > > > > > > > > would not result in any implicit casting between either > > > > > > > > > > operand, but any special arithmetic, like intermediate > > > > > > > > > > storage types or saturation handling, would be handled by > > > > > > > > > > the underlying IR? > > > > > > > > > > > > > > > > > > Yes, for operations which require precision that cannot be > > > > > > > > > provided by any of the existing types, there must be an > > > > > > > > > 'invisible' implicit conversion to a type which can represent > > > > > > > > > all of the values of either operand. This conversion cannot > > > > > > > > > be represented in the AST as it is today. > > > > > > > > > > > > > > > > > > The simplest solution is indeed to not have any implicit cast > > > > > > > > > at all in the AST and resolve these conversions when needed > > > > > > > > > (CodeGen and consteval are the locations I can think of), but > > > > > > > > > ultimately it feels a bit dirty... I think that the best > > > > > > > > > solution AST-wise is to define a completely new type class > > > > > > > > > (perhaps FullPrecisionFixedPointType) that represents a > > > > > > > > > fixed-point type with arbitrary width, scale, signedness and > > > > > > > > > saturation. Then you can define ImplicitCasts to an instance > > > > > > > > > of this type that can fit both the `int` and the `fract`. I > > > > > > > > > don't know if adding this is acceptable upstream, though. > > > > > > > > > > > > > > > > > > I think all of these rules must apply to fixed-fixed > > > > > > > > > operations as well; a `short accum * long fract` must be done > > > > > > > > > as a type that does not exist, similar to fixed-int. It's not > > > > > > > > > clear how saturation should work here either... > > > > > > > > > > > > > > > > > > I also noticed now that the spec says in regards to > > > > > > > > > comparison operators, `When comparing fixed-point values with > > > > > > > > > fixed-point values or integer values, the values are compared > > > > > > > > > directly; the values of the operands are not converted before > > > > > > > > > the comparison is made.` I'm not sure what this means. > > > > > > > > In any case, to clarify, I think there are two paths to > > > > > > > > consider. Either: > > > > > > > > > > > > > > > > - Add a new type class to the type system that encapsulates > > > > > > > > an arbitrary-precision fixed-point type that can be used for > > > > > > > > implicit casts when operating on fixed-point and integer types. > > > > > > > > This is in my opinion the cleaner solution, since it retains > > > > > > > > invariants on the types of operators and simplifies any logic > > > > > > > > that deals with operators; or, > > > > > > > > - Leave the operands of these operations uncasted. This is in > > > > > > > > some way simpler, since it doesn't require adding a whole new > > > > > > > > type, but it complicates other parts of the code. Anything that > > > > > > > > wants to deal with fixed-point operators will need to know how > > > > > > > > to do fixed-point conversion as well, which isn't a very good > > > > > > > > separation of responsibility IMO. It also breaks the C > > > > > > > > invariant of operands of arithmetic types being in a common > > > > > > > > type, which might be surprising to people. > > > > > > > > > > > > > > > > > > > > > > > I'm actually more of a fan for the second case. Aside, aside from > > > > > > > the literal parsing in NumericLieralParser, wouldn't the only > > > > > > > other place that would actually need to know about fixed point > > > > > > > conversion be `ScalarExprEmitter` under CodeGen/CGExprScalar.cpp? > > > > > > > > > > > > > > It seems that it's this class that creates the binary operations > > > > > > > and other code gen classes like CodeGenFunction just make > > > > > > > underlying calls to ScalarExprEmitter, so the actual conversion > > > > > > > logic may just be contained here. Most of the implicit casting > > > > > > > handled under UsualArithmeticConversions seems to be handled by > > > > > > > `VisitCastExpr` under ScalarExprEmitter also, so adding another > > > > > > > casting type would in the end just result in another case in the > > > > > > > switch statement there, which in turn may just result in another > > > > > > > call to ScalarExprEmitter. > > > > > > > > > > > > > > I can see how it might be weird at first that these types don't > > > > > > > fall under usual arithmetic, but the standard does specify that > > > > > > > it wouldn't. > > > > > > Regarding comparison operators, my guess is that it means during > > > > > > comparison operations specifically, the actual underlying values of > > > > > > each operand are compared instead of having the special type > > > > > > conversions take place. That is, `1.0k != 1` but `1.0k == 128` > > > > > > (assuming scale of 7). If this is the case, we could actually save > > > > > > a few operations not having to do a shift on the integer. > > > > > > > > > > > > I also can't seem to find a test case used by GCC where they > > > > > > explicitly compare a fixed point type against an integer. Normally, > > > > > > they instead assign the FP literal to an integral type, then > > > > > > compare that against another integer. > > > > > > > > > > > > I'm referring to `CONV_ACCUM_INT` in > > > > > > https://github.com/gcc-mirror/gcc/blob/e11be3ea01eaf8acd8cd86d3f9c427621b64e6b4/gcc/testsuite/gcc.dg/fixed-point/convert.h > > > > > > I'm actually more of a fan for the second case. Aside, aside from > > > > > > the literal parsing in NumericLieralParser, wouldn't the only other > > > > > > place that would actually need to know about fixed point conversion > > > > > > be ScalarExprEmitter under CodeGen/CGExprScalar.cpp? > > > > > > > > > > ExprConstant (consteval) would also have to know, since the input > > > > > expressions would be these 'unbalanced' binary operations. I'm not > > > > > sure why it would affect literal parsing, though? > > > > > > > > > > Regarding VisitCastExpr; in the first case, I'm not talking about > > > > > adding a new CastKind, I'm talking about adding a whole new type > > > > > altogether. This type would be just as much a fixed-point type as the > > > > > builtin ones, just with a configurable width and scale. Then, > > > > > something like this: > > > > > ``` > > > > > int * fract > > > > > ``` > > > > > where int is 32 bits and fract is 16 bits Q15, would become > > > > > ``` > > > > > (fract)((FullPrecFixedPoint<32+16, 0+15>)int * > > > > > (FullPrecFixedPoint<32+16, 0+15>)fract) > > > > > ``` > > > > > The cast on the `int` is a `CK_IntegerToFixedPointCast`, and the cast > > > > > on the `fract` is a `CK_FixedPointCast`. All values and operations > > > > > are self-consistent and fully representable in the AST. Converting to > > > > > and from a FullPrecFixedPoint type is no different from converting to > > > > > and from, say, `fract`. They are both fixed-point types with width > > > > > and scale; one is just built-in and the other is 'artificial'. The > > > > > multiplication is performed like any other fixed-point operation, > > > > > just in a higher width and (possibly higher) scale than either of the > > > > > operands. > > > > > > > > > > The issue I have with the second case is that the AST is somehow left > > > > > 'unfinished'. There *are* casts there, but they are just not > > > > > representable in the AST. In order to represent them, you would need > > > > > to add these arbitrary-precision types. > > > > > Regarding comparison operators, my guess is that it means during > > > > > comparison operations specifically, the actual underlying values of > > > > > each operand are compared instead of having the special type > > > > > conversions take place. That is, 1.0k != 1 but 1.0k == 128 (assuming > > > > > scale of 7). If this is the case, we could actually save a few > > > > > operations not having to do a shift on the integer. > > > > > > > > Right... That seems incredibly dangerous to me; I really hope this > > > > isn't what the spec means. 1.0 is by no means the same thing as 128. On > > > > top of that, it means that comparisons between fixed-point and integer > > > > can vary depending on the scale of the fixed-point type; this feels > > > > really shaky to me. Heck, if SameFBits is false, for a scale of 7, > > > > `0.5r == 64`, but `0.5ur != 64`. It might even be the case that `0.5r > > > > != 0.5ur`. Absolutely bizarre, and incredibly confusing for programmers! > > > > > > > > It might have been done this way to make it easy to inspect the raw > > > > bits of a fixed-point number, but why not just do a bit-preserving > > > > conversion and compare as an integer in that case? > > > > > > > > DSP-C simply prohibits 'ambiguous' type conversions such as these to > > > > prevent this confusion from happening. > > > I also noticed that the doc provides a `bitsfx` function for getting this > > > underlying integer value representation from a fixed point type. Still > > > going to leave the comparisons as what I initially interpreted them as by > > > comparing them normally (`1.0k == 1`). > > Understandable. Initially I thought it was a new cast for converting each > > fixed point type to this intermediate type. I will add this in a new > > separate patch that addresses conversions involving fixed point types since > > I feel this one is already large enough and was meant to address just > > parsing fixed point literals. > Alright. I still suspect that isn't what the standard intends. You save a few > instructions, but the comparison doesn't make sense from a value perspective. > > We have an externally written test suite which has a bunch of tests for both > DSP-C and E-C, and the E-C tests don't seem to behave the way you've > described. I've also tried to use the support in avr-gcc to determine how it > works and it doesn't seem to do a bitwise comparison there either. > > We can discuss this in later patches. > Oh, to clarify, I mean that I am performing the shifting necessary when comparing a fixed point to int or another fixed point. We are still comparing the values and not the bits directly. 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