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From: "lh_mouse at 126 dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/104632] Missed optimization about reading backwards
Date: Tue, 22 Feb 2022 09:19:24 +0000	[thread overview]
Message-ID: <bug-104632-4-7mdSVo7ibV@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-104632-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104632

--- Comment #2 from LIU Hao <lh_mouse at 126 dot com> ---
I don't think it's a duplicate. This only happens when reading through a
pointer by negative offsets. If I change the code to read by non-negative
offsets, GCC is actually very happy about it:

https://gcc.godbolt.org/z/sT9hzcndW

```
// In real code this was
//   `load_le32_backwards(::std::reverse_iterator<const unsigned char*> ptr)
unsigned
load_le32_backwards(const unsigned char* ptr)
  {
    unsigned word =    ptr[3];
    word = word << 8 | ptr[2];
    word = word << 8 | ptr[1];
    word = word << 8 | ptr[0];
    return word;
  }

```

```
load_le32_backwards(unsigned char const*):
        mov     eax, DWORD PTR [rdi]
        ret
```

  parent reply	other threads:[~2022-02-22  9:19 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-02-22  8:36 [Bug c++/104632] New: Missed optimization about backward reads lh_mouse at 126 dot com
2022-02-22  8:46 ` [Bug tree-optimization/104632] Missed optimization about reading backwards pinskia at gcc dot gnu.org
2022-02-22  9:19 ` lh_mouse at 126 dot com [this message]
2022-02-22 10:39 ` rguenth at gcc dot gnu.org

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