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From: "amonakov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/115161] [15 Regression] highway-1.0.7 miscompilation of
 some SSE2 intrinsics
Date: Sat, 25 May 2024 17:15:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115161

--- Comment #23 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
(In reply to Sergei Trofimovich from comment #22)
> Here `pcmpeqd %xmm2,%xmm1` is a problematic instruction. Why does `gcc` u=
se
> `%xmm2` (result of `cvttps2dq`) instead of, say `%xmm0` which contains
> `0xFFFFffff` pattern?

%xmm0 contains the first all-ones argument to assert_eq. The comparison nee=
ds
to happen against a register that contains 0x7fff_ffff (x4), and GCC "think=
s"
that %xmm2 contains that value. At -O1, the optimization is done by the RTL=
 CSE
pass (you can confirm by switching it off with -fdisable-rtl-cse1 along -O1=
),
it sees

(insn 6 5 7 2 (set (reg:V4SF 97)
        (mem/u/c:V4SF (symbol_ref/u:DI ("*.LC1") [flags 0x2]) [0  S16 A128]=
))
"/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/xmmintrin.h":386:19 1335
{movv4sf_internal}
     (expr_list:REG_EQUAL (const_vector:V4SF [
                (const_double:SF 4.294967296e+9 [0x0.8p+33]) repeated x4
            ])
        (nil)))

(insn 10 9 11 2 (set (reg:V4SI 98)
        (fix:V4SI (reg:V4SF 97)))
"/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include/emmintrin.h":832:19 2635
{fix_truncv4sfv4si2}
     (expr_list:REG_EQUAL (const_vector:V4SI [
                (const_int 2147483647 [0x7fffffff]) repeated x4
            ])
        (expr_list:REG_DEAD (reg:V4SF 99)
            (nil))))

Note the incorrect REG_EQUAL caused by inappropriate use of fix:V4SI where =
the
source code had cvttps2dq.=