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From: "vajdaz at protonmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/93934] Unnecessary fld of uninitialized float stack
 variable results in ub of valid C++ code
Date: Thu, 14 Oct 2021 10:05:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D93934

--- Comment #19 from Zoltan Vajda <vajdaz at protonmail dot com> ---
(In reply to Uro=C5=A1 Bizjak from comment #18)
> The following patch fixes the PR, see the comment inline:
>=20
> --cut here--
> diff --git a/gcc/config/i386/i386-expand.c b/gcc/config/i386/i386-expand.c
> index 6e2b7920d2b..b87490fe544 100644
> --- a/gcc/config/i386/i386-expand.c
> +++ b/gcc/config/i386/i386-expand.c
> @@ -4094,6 +4094,15 @@ ix86_expand_fp_movcc (rtx operands[])
>           && !TARGET_64BIT))
>      return false;
>=20=20
> +  /* Disable SFmode and DFmode x87 FCMOV with trapping math
> +     to prevent speculative load using FLDL/FLDS from uninitialized
> +     memory location, which can contain sNaN value.  FLDL/FLDS traps
> +     on sNaN, see PR93934.  */
> +
> +  if ((mode =3D=3D SFmode || mode =3D=3D DFmode)
> +      && flag_trapping_math)
> +    return false;
> +
>    /* The floating point conditional move instructions don't directly
>       support conditions resulting from a signed integer comparison.  */
>=20=20
> --cut here--

The problem does not only apply for conditional moves! I can turn on sse, f=
or
example.

https://gcc.godbolt.org/z/jP3Kne8T5

Then the problematic code with the conditional move disappears, but I have a
similar speculative fld problem in another situation.

.L10:
        inc     esi
        cmp     edi, esi
        jne     .L11
        test    bl, bl                <=3D test input variable 'b'
        fld     QWORD PTR [ebp-24]    <=3D load of (maybe) uninitialized 'r=
esult'
        je      .L24                  <=3D jump based on value of 'b'
        add     esp, 20
        pop     ebx
        pop     esi
        pop     edi
        pop     ebp
        ret
.L24:
        fstp    st(0)
.L8:
        fld     QWORD PTR .LC1
        add     esp, 20
        pop     ebx
        pop     esi
        pop     edi
        pop     ebp
        ret=