linux-kernel - Re: [PATCH misc-next] drivers/misc: remove redundant rc variable

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Message-Id: <20220105092229.617027-1-chi.minghao@zte.com.cn>
Date:   Wed,  5 Jan 2022 09:22:29 +0000
From:   cgel.zte@...il.com
To:     gregkh@...uxfoundation.org
Cc:     arnd@...db.de, cgel.zte@...il.com, chi.minghao@....com.cn,
        fkassabri@...ana.ai, linux-kernel@...r.kernel.org,
        obitton@...ana.ai, ogabbay@...nel.org, osharabi@...ana.ai,
        ynudelman@...ana.ai, zealci@....com.cn
Subject: Re: [PATCH misc-next] drivers/misc: remove redundant rc variable

I found that there are indeed some optimizations
after removing the redundancy.

For example:

int foo(int a, int b)
{
        int ret;
        ret = a>b ? a: b;
        return ret;
}
int main()
{
        int val;
        val = foo(7,2);
        return 0;
}

and

int foo(int a, int b)
{
        return a>b ? a: b;
}
int main()
{
        int val;
        val = foo(7,2);
        return 0;
}

The corresponding disassembly code is as follows：

    1129:       f3 0f 1e fa             endbr64 
    112d:       55                      push   %rbp
    112e:       48 89 e5                mov    %rsp,%rbp
    1131:       89 7d fc                mov    %edi,-0x4(%rbp)
    1134:       89 75 f8                mov    %esi,-0x8(%rbp)
    1137:       8b 45 fc                mov    -0x4(%rbp),%eax
    113a:       39 45 f8                cmp    %eax,-0x8(%rbp)
    113d:       0f 4d 45 f8             cmovge -0x8(%rbp),%eax
    1141:       5d                      pop    %rbp
    1142:       c3                      retq

and

    1129:       f3 0f 1e fa             endbr64 
    112d:       55                      push   %rbp
    112e:       48 89 e5                mov    %rsp,%rbp
    1131:       89 7d ec                mov    %edi,-0x14(%rbp)
    1134:       89 75 e8                mov    %esi,-0x18(%rbp)
    1137:       8b 45 ec                mov    -0x14(%rbp),%eax
    113a:       39 45 e8                cmp    %eax,-0x18(%rbp)
    113d:       0f 4d 45 e8             cmovge -0x18(%rbp),%eax
    1141:       89 45 fc                mov    %eax,-0x4(%rbp)
    1144:       8b 45 fc                mov    -0x4(%rbp),%eax
    1147:       5d                      pop    %rbp
    1148:       c3                      retq

After removing the redundancy, the compiler does
have some optimizations