https://bugs.llvm.org/show_bug.cgi?id=40776
Bug ID: 40776
Summary: missed optimization with C constructors of sparse
structures
Product: clang
Version: 8.0
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: -New Bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
I stumbled over a file with excessive stack usage in the Linux kernel when
building with clang, but not with gcc, reduced the test case to:
struct i2c_adapter {
char name[48];
int a[1000];
} hexium_attach_hexium;
void hexium_attach(void) {
hexium_attach_hexium = (struct i2c_adapter){"hexium gemini"};
}
$ clang-8 -Werror -c hexium-gemini.c -Wframe-larger-than=100 -O2
hexium-gemini.c:5:6: error: stack frame size of 4056 bytes in function
'hexium_attach' [-Werror,-Wframe-larger-than=]
Comparing the output shows that clang not only allocates stack space for the
extra copy of the structure, but also fails to optimize the string assignment:
https://godbolt.org/z/7jpy_y
I worked around it in the kernel sources by using an explict memset and
strscpy, but this seems like a generally useful optimization that could be
added.
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