https://bugs.llvm.org/show_bug.cgi?id=40226
Bug ID: 40226
Summary: Passing struct by value instead of const& results in
linear stack bloat
Product: clang
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: C++
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
Created attachment 21293
--> https://bugs.llvm.org/attachment.cgi?id=21293&action=edit
A small repro
Vladislav Kuzkokov has reported the following problem with trunk LLVM (it also
appears to date back to at least LLVM 4.0.1).
When the attached program is compiled with -DBY_VALUE (which results in Struct
being passed by value, as opposed to passing a const reference) Clang allocates
extra stack space for each Struct instance in bar():
$ clang++ byval.cc -c -fno-exceptions -O2 -DBY_VALUE && objdump -Sr byval.o
...
0000000000000000 <_Z3barv>:
0: 53 push %rbx
1: 48 83 ec 20 sub $0x20,%rsp
5: 48 8d 5c 24 18 lea 0x18(%rsp),%rbx
a: 48 89 df mov %rbx,%rdi
...
$ clang++ byval.cc -c -fno-exceptions -O2 -UBY_VALUE && objdump -Sr byval.o
...
0000000000000000 <_Z3barv>:
0: 53 push %rbx
1: 48 83 ec 10 sub $0x10,%rsp
5: 48 8d 5c 24 08 lea 0x8(%rsp),%rbx
Looking at the IR, perhaps the lifetime intrinsics are missing?
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