rsmith added inline comments.
================
Comment at: clang/test/SemaCXX/rounding-math.cpp:9
+
+constexpr int f(int n) { return int(n * (1.0 / 3.0)); }
+
----------------
rsmith wrote:
> sepavloff wrote:
> > This code requires additional solution. The function body is built using
> > dynamic rounding mode, which breaks its constexprness. We can avoid this
> > kind of errors if we assume that body of a constexpr function is parsed
> > using constant rounding mode (in this example it is the default mode). It
> > makes parsing constexpr function different from non-constexpr ones, but
> > enables convenient use:
> > ```
> > constexpr int add(float x, float y) { return x + y; }
> >
> > #pragma STDC FENV_ROUND FE_UPWARD
> > int a2 = add(2.0F, 0x1.000002p0F);
> >
> > #pragma STDC FENV_ROUND FE_DOWNWARD
> > int a3 = add(2.0F, 0x1.000002p0F);
> > ```
> > If calls of constexpr functions are processes with FP options acting in the
> > call site, a call to constexpr function becomes equivalent to execution of
> > statements of its body.
> I don't understand what you mean by "breaks its constexprness". Using a
> dynamic rounding mode for the body of the function is exactly what we want
> here. When the function is called in a manifestly constant evaluated context,
> we should use the default rounding mode, and when it's called at runtime, we
> use the appropriate dynamic rounding mode; if we try to constant evaluate it
> outside of a manifestly constant evaluated constant, we deem it non-constant.
>
> When `constexpr` function is called at runtime, it's a completely normal
> function with normal semantics. It would be wrong to use the default rounding
> mode when parsing its body.
To be clear: in your example with `add`, both `a2` and `a3` should be
initialized to the same value, which should be rounded with the default
rounding mode. Per ISO18661, `FENV_ROUND` only affects operations in its
lexical scope, not functions called within those operations, and per the
regular C++ rules, `constexpr` functions behave like regular functions, not
like macros.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D89360/new/
https://reviews.llvm.org/D89360
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