• d-bugmail puremagic.com (66/66) Oct 26 2017 https://issues.dlang.org/show_bug.cgi?id=17938

d-bugmail puremagic.com writes:

https://issues.dlang.org/show_bug.cgi?id=17938

          Issue ID: 17938
           Summary: Detect immutable variadic arguments
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: performance
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: dfj1esp02 sneakemail.com

---
void f(in string[] a...);
void g()
{
        f("a","b","c");
}
void h()
{
        static immutable string[3] b=["a","b","c"];
        f(b);
}
---
void example.g():
 push   %rbp
 mov    %rsp,%rbp
 sub    $0x40,%rsp
 mov    %rbx,-0x38(%rbp)
 mov    $0x0,%ecx
 mov    $0x1,%eax
 mov    %rax,-0x30(%rbp)
 mov    %rcx,-0x28(%rbp)
 mov    $0x0,%edx
 mov    $0x1,%ebx
 mov    %rbx,-0x20(%rbp)
 mov    %rdx,-0x18(%rbp)
 mov    $0x0,%ecx
 mov    $0x1,%esi
 mov    %rsi,-0x10(%rbp)
 mov    %rcx,-0x8(%rbp)
 lea    -0x30(%rbp),%rdx
 mov    $0x3,%edi
 mov    %rdx,%rsi
 xor    %eax,%eax
 callq  55 <void example.g()+0x55>
 mov    -0x38(%rbp),%rbx
 leaveq 
 retq   
void example.h():
 push   %rbp
 mov    %rsp,%rbp
 mov    $0x0,%edx
 mov    $0x3,%edi
 mov    %rdx,%rsi
 xor    %eax,%eax
 callq  18 <void example.h()+0x18>
 pop    %rbp
 retq   
---
The calls are equivalent, but function h looks more efficient and has less code
generated. Function g amounts to 24 instructions and 91 bytes, h - to 9
instructions and 32 bytes.

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