Re: [PATCH] alpha: round to odd before narrowing TFmode to SFmode

On 8/9/2026 1:12 PM, Matt Turner wrote:
> Alpha has no instruction converting TFmode to SFmode, so trunctfsf2 goes
> through DFmode and has to avoid rounding twice.
>
> It tried to, by setting a sticky bit at fraction bit 48 when the low word
> of the TFmode value was nonzero: a DFmode value with that bit set cannot
> sit exactly halfway between two SFmode values, leaving the second
> rounding nothing to break a tie on.  But the bit is set before the
> conversion to DFmode, and the carry out of the bits that conversion
> discards can propagate up through bit 48 and clear it, landing on the
> halfway value it was there to avoid.
>
> Round to odd at the last fraction bit DFmode keeps instead.  The value is
> then exactly representable in DFmode, so that conversion does not round
> and the one to SFmode is the only rounding.
>
> Over values constructed around SFmode halfway points the old sequence is
> wrong for about a quarter of them and the new one for none.  Uniformly
> random values do not reach it: that needs the conversion to DFmode to
> carry all the way to a halfway value.
>
> Rounding twice also loses the underflow flag when the result is subnormal.
> glibc's narrowing functions compute a round-to-odd value in the wider type
> and convert it once, and that single conversion is what is meant to raise
> underflow; splitting it in two can leave the intermediate DFmode value
> exactly representable in SFmode, so the final step raises nothing.
>
> Found through glibc's math testsuite, where three of the narrowing fma
> tests returned a wrong result.  Retested there on an EV67 with a glibc
> built by a compiler carrying this patch: the wrong results are gone, every
> test now reporting a maximum error of 0 ulp, and the six narrowing add and
> subtract tests that had been failing on the lost underflow flag pass.  The
> narrowing tests that still fail there do so for an unrelated reason, Alpha
> determining tininess from the delivered result rather than as IEEE 754
> describes, which no change to the compiler can address.
> ---
>   gcc/config/alpha/alpha.md                     | 23 +++++++++++------
>   gcc/testsuite/gcc.target/alpha/trunctfsf2-1.c | 25 +++++++++++++++++++
>   2 files changed, 40 insertions(+), 8 deletions(-)
>   create mode 100644 gcc/testsuite/gcc.target/alpha/trunctfsf2-1.c
THanks.  I constructed a ChangeLog entry and pushed this to the trunk.

Not sure how much time you have Matt, but these tests started failing 
about a month ago on alpha-linux-gnu.  No idea why yet.

Tests that now fail, but worked before (28 tests):

gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -O0  execution test
gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -O0  execution test
gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -O1  execution test
gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -O1  execution test
gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -O2  execution test
gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -O2  execution test
gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  execution test
gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  execution test
gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  execution test
gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  execution test
gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -O3 -g  execution test
gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -O3 -g  execution test
gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -Os  execution test
gcc: gcc.dg/torture/fp-int-convert-float32-timode.c   -Os  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -O0  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -O0  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -O1  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -O1  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -O2  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -O2  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -O3 -g  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -O3 -g  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -Os  execution test
gcc: gcc.dg/torture/fp-int-convert-timode.c   -Os  execution test


31d967232a9331ad72571da27550b2f5d7d38523 was the last good hash.  f06e56614b948ad88dabf63020f47713b7dea0ec was the first reported bad hash if you're going to try and chase it down.



Jeff