[PATCH] D27898: [compiler-rt] [builtins] Implement __floattitf() & __floatuntitf()

[Steve Canon via Phabricator via cfe-commits]

scanon added inline comments.


================
Comment at: lib/builtins/floattitf.c:65
+        if (a & ((tu_int)1 << LDBL_MANT_DIG)) {
+            a >>= 1;
+            ++e;
----------------
mgorny wrote:
> 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`
This sounds like you removed the exponent adjustment (`++e`) as well as (or instead of) the shift (`a >>= 1`). Without seeing the change, I can't be certain, of course.


https://reviews.llvm.org/D27898