ebevhan added a comment. Just a couple more comments and then I think it looks good.
We can discuss the conversion and comparison issues in later patches. ================ Comment at: include/clang/AST/ASTContext.h:1951 + unsigned char getFixedPointScale(const QualType &Ty) const; + unsigned char getFixedPointIBits(const QualType &Ty) const; + ---------------- These can probably take `QualType` directly. ================ Comment at: include/clang/Basic/TargetInfo.h:99 + // sign if SameFBits is set. + unsigned char ShortAccumFBits; + unsigned char AccumFBits; ---------------- Could these and their accessors be called 'Scale' like in ASTContext? Only a consistency nit. ================ Comment at: lib/Sema/SemaExpr.cpp:1248 + bool RHSFixed = RHSType->isFixedPointType(); + + if (LHSFixed && RHSFixed) { ---------------- 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. Repository: rC Clang https://reviews.llvm.org/D46915 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits