ebevhan added inline comments.
================ Comment at: lib/Sema/SemaExpr.cpp:1248 + bool RHSFixed = RHSType->isFixedPointType(); + + if (LHSFixed && RHSFixed) { ---------------- 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. ================ 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: > > > > > > 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. 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