On Wednesday 14 October 2015 08:51:11 Thiago Macieira wrote:
> The separation of the string itself from the size and the d pointer allows
> the compiler, if it wants to, to share strings. In fact, disassembly of
>
> f(QStringLiteral("foo"), QStringLiteral("foo"))
>
> produces one copy of u"foo" only.
Let me expand on this.
Current Qt5 QStringLiteral("foo") produces a data block of size
sizeof(QArrayData) + sizeof(u"foo") = 24 + 8 = 32 bytes
and the fact that QStringLiterals don't share will cause the innocent-looking
above code require 64 bytes of read-only data.
My current code expands to 8 bytes of read-only data, at the expense of a
little more code.
Current code:
leaq _ZZZ1fvENKUlvE0_clEvE15qstring_literal(%rip), %rax
movq %rax, 16(%rsp)
leaq _ZZZ1fvENKUlvE_clEvE15qstring_literal(%rip), %rax
movq %rax, (%rsp)
; followed by the call:
movq %rsp, %rdi
leaq 16(%rsp), %rsi
call _Z1fRK7QStringS1_@PLT
My code:
leaq .LC0(%rip), %rcx ; u"foo"
movq _ZN10QArrayData18shared_static_dataE@GOTPCREL(%rip), %rax
movq %rcx, 40(%rsp)
movl $3, 48(%rsp)
movq %rcx, 8(%rsp)
movq %rax, 32(%rsp)
movq %rax, (%rsp)
movl $3, 16(%rsp)
; the call itself is unchanged:
movq %rsp, %rdi
leaq 32(%rsp), %rsi
call _Z1fRK7QStringS1_@PLT
--
Thiago Macieira - thiago.macieira (AT) intel.com
Software Architect - Intel Open Source Technology Center
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