On Thu, Sep 23, 2021 at 10:44 PM Patrick Palka via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> In encode_ieee_double/quad, the assignment
>
> unsigned long VAL = r->sign << 31;
>
> is intended to set the 31st bit of VAL whenever the given REAL_CST is
> negative. But on LP64 hosts it also unintentionally sets the upper 32
> bits of VAL due to the promotion of r->sign from unsigned:1 to int and
> the subsequent sign extension of the shifted value from int to long.
>
> In the C++ frontend, this bug causes incorrect mangling of negative
> double values due to the output of real_to_target during write_real_cst
> unexpectedly having the upper 32 bits of each word set. (I'm not sure
> if/how this bug manifests itself outside of the frontend..)
>
> This patch fixes this by avoiding the unwanted sign extension. Note
> that r0-53976 fixed the same bug in encode_ieee_single long ago.
>
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk and perhaps the release branches?
OK for trunk and branches.
Thanks,
Richard.
> PR c++/98216
> PR c++/91292
>
> gcc/ChangeLog:
>
> * real.c (encode_ieee_double): Avoid incorrect sign extension.
> (encode_ieee_quad): Likewise.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp2a/nontype-float2.C: New test.
> ---
> gcc/real.c | 6 ++++--
> gcc/testsuite/g++.dg/cpp2a/nontype-float2.C | 13 +++++++++++++
> 2 files changed, 17 insertions(+), 2 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-float2.C
>
> diff --git a/gcc/real.c b/gcc/real.c
> index 555cf44c142..8c7a47a69e6 100644
> --- a/gcc/real.c
> +++ b/gcc/real.c
> @@ -3150,9 +3150,10 @@ encode_ieee_double (const struct real_format *fmt,
> long *buf,
> const REAL_VALUE_TYPE *r)
> {
> unsigned long image_lo, image_hi, sig_lo, sig_hi, exp;
> + unsigned long sign = r->sign;
> bool denormal = (r->sig[SIGSZ-1] & SIG_MSB) == 0;
>
> - image_hi = r->sign << 31;
> + image_hi = sign << 31;
> image_lo = 0;
>
> if (HOST_BITS_PER_LONG == 64)
> @@ -3938,10 +3939,11 @@ encode_ieee_quad (const struct real_format *fmt, long
> *buf,
> const REAL_VALUE_TYPE *r)
> {
> unsigned long image3, image2, image1, image0, exp;
> + unsigned long sign = r->sign;
> bool denormal = (r->sig[SIGSZ-1] & SIG_MSB) == 0;
> REAL_VALUE_TYPE u;
>
> - image3 = r->sign << 31;
> + image3 = sign << 31;
> image2 = 0;
> image1 = 0;
> image0 = 0;
> diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-float2.C
> b/gcc/testsuite/g++.dg/cpp2a/nontype-float2.C
> new file mode 100644
> index 00000000000..5db208a05d1
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/nontype-float2.C
> @@ -0,0 +1,13 @@
> +// PR c++/98216
> +// { dg-do compile { target c++20 } }
> +
> +template<auto> void f() { }
> +
> +template void f<-1.0f>();
> +template void f<-2.0f>();
> +
> +template void f<-1.0>();
> +template void f<-2.0>();
> +
> +template void f<-1.0L>();
> +template void f<-2.0L>();
> --
> 2.33.0.514.g99c99ed825
>
