https://llvm.org/bugs/show_bug.cgi?id=30638
Bug ID: 30638
Summary: missing optimization (alias analysis issue?)
Product: new-bugs
Version: 3.9
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
The following code:
#include <vector>
#include <memory>
struct base
{
virtual ~base()
{}
};
void f(std::vector<std::unique_ptr<base> >& v)
{
v.back().release();
v.pop_back();
}
GCC 6.2 is able to optimize to just two instructions:
sub QWORD PTR [rdi+8], 8
ret
The code generated by clang 3.9.0 is less efficient:
push rbx
mov rax, qword ptr [rdi + 8]
mov qword ptr [rax - 8], 0
mov rax, qword ptr [rdi + 8]
lea rbx, [rax - 8]
mov qword ptr [rdi + 8], rbx
mov rdi, qword ptr [rax - 8]
test rdi, rdi
je .LBB0_2
mov rax, qword ptr [rdi]
call qword ptr [rax + 8]
.LBB0_2:
mov qword ptr [rbx], 0
pop rbx
ret
As [rax - 8] is reloaded after subtraction I tend believe it is an alias
analysis issue.
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