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From: Paul Edwards <mutazilah@gmail.com>
To: GCC Development <gcc@gcc.gnu.org>
Subject: gcc 3.2.3 x64 negative indexes
Date: Wed, 7 Feb 2024 23:02:51 +0800	[thread overview]
Message-ID: <CAMi4NxaOSAhwnNTkbOiC7nz_0GfF72e9uTszs6HNTAhwqh5MMA@mail.gmail.com> (raw)

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Hi.

I am using a slightly modified gcc 3.2.3 for x86_64 and for this code:

int fff(char *x)
{
return (x[-1]);
}


It is generating:

.globl fff
fff:
.LFB2:
        movl    $4294967295, %eax
        movsbl  (%rax,%rcx),%eax
        ret


My understanding is that that move of -1 into eax
does NOT sign-extend, and thus results in a very
high index, which causes a crash.

I may have stuffed something up myself.

Anyone know which code governs this behavior?

Thanks. Paul.

             reply	other threads:[~2024-02-07 15:03 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-02-07 15:02 Paul Edwards [this message]
2024-02-07 15:12 ` Jakub Jelinek
2024-02-09 21:34   ` Paul Edwards
2024-02-09 21:38     ` Paul Edwards
2024-02-10 13:41       ` Paul Edwards
2024-02-10 17:42         ` Joe Monk
2024-02-10 19:48         ` Paul Edwards
2024-02-11  0:03         ` Paul Edwards

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