hubert.reinterpretcast added inline comments.
================ Comment at: clang/lib/AST/ExprConstant.cpp:12452 + int Ilogb; + if (APFloat::opStatus St = ilogb(F, Ilogb); !isConstantOpStatus(St)) + return false; ---------------- hubert.reinterpretcast wrote: > hubert.reinterpretcast wrote: > > Izaron wrote: > > > majnemer wrote: > > > > Izaron wrote: > > > > > jcranmer-intel wrote: > > > > > > Izaron wrote: > > > > > > > aaron.ballman wrote: > > > > > > > > jcranmer-intel wrote: > > > > > > > > > `long double` is `ppc_fp128` on at least some PPC targets, > > > > > > > > > and while I'm not entirely certain of what `ilogb` properly > > > > > > > > > returns in the corner cases of the `ppc_fp128`, I'm not > > > > > > > > > entirely confident that the implementation of `APFloat` is > > > > > > > > > correct in those cases. I'd like someone with PPC background > > > > > > > > > to comment in here. > > > > > > > > Paging @hubert.reinterpretcast for help finding a good person > > > > > > > > to comment on the PPC questions. > > > > > > > @jcranmer-intel constexpr evaluation is quite > > > > > > > machine-/target-independent. Clang evaluates it based on its > > > > > > > **internal** representation of float variables. > > > > > > > > > > > > > > [[ > > > > > > > https://github.com/llvm/llvm-project/blob/2e5bf4da99a2f8d3d4bb4f1a4d1ed968a01e8f02/llvm/include/llvm/ADT/APFloat.h#L1256 > > > > > > > | int ilogb ]] uses `Arg.getIEEE()`, that [[ > > > > > > > https://github.com/llvm/llvm-project/blob/2e5bf4da99a2f8d3d4bb4f1a4d1ed968a01e8f02/llvm/include/llvm/ADT/APFloat.h#L819-L825 > > > > > > > | returns Clang's internal float representation ]]. > > > > > > > > > > > > > > Whichever float semantics is being used, [[ > > > > > > > https://github.com/llvm/llvm-project/blob/2e5bf4da99a2f8d3d4bb4f1a4d1ed968a01e8f02/llvm/lib/Support/APFloat.cpp#L54-L61 > > > > > > > | minExponent and maxExponent are representable as > > > > > > > APFloatBase::ExponentType ]], which is `int32_t`: > > > > > > > ``` > > > > > > > /// A signed type to represent a floating point numbers unbiased > > > > > > > exponent. > > > > > > > typedef int32_t ExponentType; > > > > > > > ``` > > > > > > > > > > > > > > We already use `int ilogb` in some constexpr evaluation code: [[ > > > > > > > https://github.com/llvm/llvm-project/blob/2e5bf4da99a2f8d3d4bb4f1a4d1ed968a01e8f02/clang/lib/AST/ExprConstant.cpp#L14592 > > > > > > > | link ]], it is working correct because it is working on > > > > > > > Clang's float representations. > > > > > > > We already use `int ilogb` in some constexpr evaluation code: [[ > > > > > > > https://github.com/llvm/llvm-project/blob/2e5bf4da99a2f8d3d4bb4f1a4d1ed968a01e8f02/clang/lib/AST/ExprConstant.cpp#L14592 > > > > > > > | link ]], it is working correct because it is working on > > > > > > > Clang's float representations. > > > > > > > > > > > > `APFloat::getIEEE()`, if I'm following it correctly, only returns > > > > > > the details of the high double in `ppc_fp128` floats, and I'm not > > > > > > sufficiently well-versed in the `ppc_fp128` format to establish > > > > > > whether or not the low double comes into play here. glibc seems to > > > > > > think that the low double comes into play in at least one case: > > > > > > https://github.com/bminor/glibc/blob/30891f35fa7da832b66d80d0807610df361851f3/sysdeps/ieee754/ldbl-128ibm/e_ilogbl.c > > > > > Thanks for the link to the glibc code! It helped me to understand the > > > > > IEEE754 standard better. > > > > > > > > > > I did some research and it seems like AST supports a fixed set of > > > > > float types, each working good with `ilogb`: > > > > > ``` > > > > > half (__fp16, only for OpenCL), float16, float, double, long double, > > > > > float128 > > > > > ``` > > > > > [[ > > > > > https://github.com/llvm/llvm-project/blob/7846d590033e8d661198f4c00f56f46a4993c526/clang/lib/Sema/SemaExpr.cpp#L3911-L3931 > > > > > | link to SemaExpr.cpp ]] > > > > > > > > > > It means that the constant evaluator doesn't deal with other float > > > > > types including `ppc_fp128`. > > > > > If `ppc_fp128` was supported on the AST level, it would anyway come > > > > > through type casting, and `__builtin_ilog<SUFFIX>` would deal with a > > > > > value of a known type. > > > > > > > > > > I checked the list of builtins - each builtin argument of float type > > > > > also supports only common float types: > > > > > [[ > > > > > https://github.com/llvm/llvm-project/blob/7846d590033e8d661198f4c00f56f46a4993c526/clang/include/clang/Basic/Builtins.def#L27-L31 > > > > > | link to Builtins.def 1 ]] > > > > > [[ > > > > > https://github.com/llvm/llvm-project/blob/7846d590033e8d661198f4c00f56f46a4993c526/clang/include/clang/Basic/Builtins.def#L53-L54 > > > > > | link to Builtins.def 2 ]] > > > > Won't long double map to ppc_fp128 for some targets? > > > Hi! It will map, but only **after** all the constant (constexpr) > > > calculations are done (that is, after the AST parsing stage) - in the > > > Codegen stage. > > > > > > The Clang's constant evaluator itself never deals with ppc_fp128 and > > > doesn't care about the target. > > > While parsing the AST, the constant evaluator works on the same level > > > with it, providing calculated values to the AST being built on-the-fly. > > > At the moment AST is built, constant evaluation is over. > > > The evaluator is target-independent and uses the internal representation > > > for `long double`, in the form of emulated **80-bit (x86) format**. > > > > > > The Codegen can map the AST's `long double` to `ppc_fp128` on some > > > targets. > > > It doesn't cause problems because x87 80-bit float is convertible to > > > ppc_fp128 without precision loss. > > > But the constexpr `long double` values itself were calculated using the > > > Clang's 80-bit format emulation, before the Codegen stage. > > > > > > I'm sorry if I'm not describing it clearly. It's important to me that > > > everyone understands what the trick is =) > > > So, the constant evaluator does everything with 80-bit floats and at the > > > end they can be mapped on ppc_fp128 floats if the target requires it. > > That's kind-of terrible, but at least that means that what ilogb can do > > within the scope of this patch is very clear. > > I am surprised to find that ilogb({2, -0x1p-64}) seems to return 1 on > > ppc64le Linux. > > > Of course I should have used `ilogbl`... > So, yes, ilogbl({2, -0x1p-64}) returns 0 on ppc64le Linux. @Izaron, > The evaluator is target-independent and uses the internal representation for > long double, in the form of emulated 80-bit (x86) format. This the internal representation still 80-bit for platforms where long double uses the IEEE binary 128-bit format? 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