https://llvm.org/bugs/show_bug.cgi?id=26990
Bug ID: 26990
Summary: -fno-builtin and -fno-builtin-foo disable
optimizations that introduce builtin calls (e.g., LIR)
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: Common Code Generator Code
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected]
Classification: Unclassified
The -fno-builtin option says don't recognize built-in functions that do not
begin with __builtin__ as a prefix. This is done to prevent the compiler from
optimizing such calls in the original program. However, it isn't clear to me
(or Duncan or Bob) that this should prevent the compiler from introducing new
calls to builtin functions.
For example, this bit of code is recognized by the loop idiom pass as a memset:
void foo(int *arr) {
for (unsigned i = 0; i < 8192; ++i)
arr[i] = 0;
}
When compiling with gcc, the -fno-builtin and -fno-builtin-memset options don't
prevent gcc from converting this loop into a memset:
$> aarch64-linux-gnu-gcc -O3 test.c -o - -S -fno-builtin-memset | grep memset
b memset
However, the same is not true when compiling with LLVM.
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