Changeset 203911 in webkit

Timestamp:

Jul 29, 2016, 11:52:53 AM (9 years ago)

Author:

mark.lam@apple.com

Message:

Make StringView capable of being passed or returned in only 2 registers.
​https://bugs.webkit.org/show_bug.cgi?id=160344

Reviewed by Geoffrey Garen.

We just need to #if out copy and move constructors and assignment operators.

After this change, the following test code:

JS_EXPORT_PRIVATE StringView returnStringView(StringView sv)
{

return sv;

}

... compiles to the following for x86_64:

ZN3JSC16returnStringViewEN3WTF10StringViewE:
000000000093fb20 pushq %rbp
000000000093fb21 movq %rsp, %rbp
000000000093fb24 movq %rdi, %rax Copy from arg word 0 to ret word 0.
000000000093fb27 movq %rsi, %rdx Copy from arg word 1 to ret word 1.
000000000093fb2a popq %rbp
000000000093fb2b retq

... and this for arm64:

ZN3JSC16returnStringViewEN3WTF10StringViewE:
0000000000818504 ret arg word 0 and 1 are in the same regs as ret word 0 and 1.

• wtf/text/StringView.h:

(WTF::StringView::StringView):
(WTF::StringView::~StringView):
(WTF::StringView::operator=):

Location:

trunk/Source/WTF

Files:

• ChangeLog (modified) (1 diff)
• wtf/text/StringView.h (modified) (4 diffs)

trunk/Source/WTF/ChangeLog

@@ +1 @@
+2016-07-29  Mark Lam  <mark.lam@apple.com>
+
+        Make StringView capable of being passed or returned in only 2 registers.
+        https://bugs.webkit.org/show_bug.cgi?id=160344
+
+        Reviewed by Geoffrey Garen.
+
+        We just need to #if out copy and move constructors and assignment operators.
+
+        After this change, the following test code:
+
+            JS_EXPORT_PRIVATE StringView returnStringView(StringView sv)
+            {
+                return sv;
+            }
+
+        ... compiles to the following for x86_64:
+
+            __ZN3JSC16returnStringViewEN3WTF10StringViewE:
+            000000000093fb20        pushq   %rbp
+            000000000093fb21        movq    %rsp, %rbp
+            000000000093fb24        movq    %rdi, %rax // Copy from arg word 0 to ret word 0.
+            000000000093fb27        movq    %rsi, %rdx // Copy from arg word 1 to ret word 1.
+            000000000093fb2a        popq    %rbp
+            000000000093fb2b        retq
+
+        ... and this for arm64:
+
+            __ZN3JSC16returnStringViewEN3WTF10StringViewE:
+            0000000000818504        ret // arg word 0 and 1 are in the same regs as ret word 0 and 1.
+
+        * wtf/text/StringView.h:
+        (WTF::StringView::StringView):
+        (WTF::StringView::~StringView):
+        (WTF::StringView::operator=):
+
 2016-07-29  Csaba Osztrogonác  <ossy@webkit.org>
 

trunk/Source/WTF/wtf/text/StringView.h

@@ -49 +49 @@
 
 // StringView is a non-owning reference to a string, similar to the proposed std::string_view.
-// Whether the string is 8-bit or 16-bit is encoded in the upper bit of the length member.
-// This means that strings longer than 2 gigacharacters cannot be represented.
 
 class StringView {
 public:
     StringView();
+#if CHECK_STRINGVIEW_LIFETIME
     ~StringView();
     StringView(StringView&&);
@@ -60 +59 @@
     StringView& operator=(StringView&&);
     StringView& operator=(const StringView&);
+#endif
 
     StringView(const String&);
@@ -203 +203 @@
 }
 
+#if CHECK_STRINGVIEW_LIFETIME
 inline StringView::~StringView()
 {
@@ -259 +260 @@
     return *this;
 }
+#endif // CHECK_STRINGVIEW_LIFETIME
 
 inline void StringView::initialize(const LChar* characters, unsigned length)