mgorny added inline comments.
================
Comment at: lib/builtins/floattitf.c:65
+ if (a & ((tu_int)1 << LDBL_MANT_DIG)) {
+ a >>= 1;
+ ++e;
----------------
scanon wrote:
> mgorny wrote:
> > scanon wrote:
> > > Strictly speaking there's no need to adjust `a` here. If we rounded up
> > > into a new binade, then `a` is necessarily `0b1000...0`, and the leading
> > > 1 bit will get killed by the mask when we assemble `fb.u.high.all`
> > > regardless of its position. Same comment applies to floatuntitf.
> > I'm sorry but I don't feel confident changing that. AFAIU if the
> > LDBL_MANT_DIG+1 bit is set, this code shifts it lower, so it won't actually
> > be killed by the mask.
> In binary128, as in all IEEE 754 binary interchange format encodings, the
> leading bit of the significand is implicit. The only way to end up in this
> code path is `0b111...1` rounding up to `0b100...00`, meaning that the
> significand is 1.0, which is stored as all-zeros (i.e. the leading bit is
> necessarily masked).
>
> To be more explicit, LDBL_MANT_DIG is 113. If this shift happens, after the
> shift bit 112 is set, and bits 111:0 are zero. The mask `((a >> 64) &
> 0x0000ffffffffffffLL)` discards bit 112 (= 64 + 48).
Well, I've tried removing this and it causes one of the tests to fail:
`error in __floatuntitf(0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF) = 0X1P+127,
expected 0X1P+128`
https://reviews.llvm.org/D27898
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