andrew.w.kaylor added a comment.
Can you update the clang user's manual to describe the behavior of this option?
The excess precision issue is also mentioned in the clang language extensions
document
(https://clang.llvm.org/docs/LanguageExtensions.html#half-precision-floating-point).
================
Comment at: clang/include/clang/Basic/LangOptions.def:320
BENIGN_ENUM_LANGOPT(FPEvalMethod, FPEvalMethodKind, 2, FEM_UnsetOnCommandLine,
"FP type used for floating point arithmetic")
+BENIGN_ENUM_LANGOPT(FPPrecisionMode, FPExcessPrecisionModeKind, 2,
FPP_Standard, "FP precision used for floating point arithmetic")
LANGOPT(NoBitFieldTypeAlign , 1, 0, "bit-field type alignment")
----------------
A lot of people mix up precision and accuracy, so I think this description is
likely to be ambiguous. I'd suggest "Intermediate truncation behavior for
floating point arithmetic"
================
Comment at: clang/include/clang/Basic/LangOptions.h:300
+ FPP_Fast,
+ FPP_None
+ };
----------------
Is FPP_None somehow the same as fexcess-precision=16? If not, what does it mean?
================
Comment at: clang/include/clang/Driver/Options.td:1564
+def fexcess_precision_EQ : Joined<["-"], "fexcess-precision=">,
Group<f_Group>, Flags<[CC1Option]>,
+ HelpText<"Specifies the precision in which this floating-point operations
will be calculated.">,
+ Values<"standard,fast,16">, NormalizedValuesScope<"LangOptions">,
----------------
Again, I think this is ambiguous. My understanding is that the option is more
about the rules for when intermediate values are truncated to source precision
than when what precision is prescribed for the calculation. For targets that
have native support for half precision types, the calculations should always be
done at source precision and this option will have no effect. It's only when
the native ISA doesn't support the source precision that this is needed and
even then we will always perform calculations at the nearest precision to
source precision that is available, right?
================
Comment at: clang/lib/Driver/ToolChains/Clang.cpp:2995
+ StringRef Val = A->getValue();
+ if (TC.getTriple().getArch() == llvm::Triple::x86 && Val.equals("16"))
+ D.Diag(diag::err_drv_unsupported_opt_for_target)
----------------
Why is 16 only supported for x86? Is it only here for gcc compatibility?
================
Comment at: clang/test/CodeGen/X86/fexcess-precise.c:89
+// CHECK-EXT-NEXT: [[EXT1:%.*]] = fpext half [[TMP1]] to float
+// CHECK-EXT-NEXT: [[MUL:%.*]] = fmul float [[EXT]], [[EXT1]]
+// CHECK-EXT-NEXT: [[TMP2:%.*]] = load half, ptr [[C_ADDR]], align 2
----------------
This is not what I'd expect to see. I'd expect the operations to use the half
type with explicit truncation inserted where needed.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D136176/new/
https://reviews.llvm.org/D136176
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