http://llvm.org/bugs/show_bug.cgi?id=12686
Bug #: 12686
Summary: useless memcpy when returning struct into local
variable
Product: clang
Version: trunk
Platform: PC
OS/Version: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: -New Bugs
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Classification: Unclassified
Returning a struct directly into a local variable generates poor code. The
following C code generates a call to memcpy on x86-64:
struct big { int a[50]; };
struct big f(void);
void g(struct big *p};
void h(void) {
struct big b; /**/
b = f(); /**/
g(&b);
}
The memcpy _is_ optimised away if the local variable is initialised by
replacing the two marked lines with:
struct big b = f();
This also makes the stack frame smaller, since there is no need for a separate
return buffer. I see no reason why the two versions should not yield identical
code.
clang version 3.1 (trunk 154679)
Target: x86_64-unknown-linux-gnu
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