https://bugs.llvm.org/show_bug.cgi?id=42439
Bug ID: 42439
Summary: __int128 stack calling conventions are incorrect on
x86-64
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
This is a bug report that was posted internally at Google, since the user can't
register here on bugs.llvm.org:
Clang violates x86-64 calling convention in the obscure case when __int128 is
passed on stack.
Conside the following two functions:
__int128 foo(__int128 x, __int128 y, __int128 z, uint64_t a, __int128 c) {
return x + y + z + a + c;
}
__int128 foo(__int128 x, __int128 y, __int128 z, uint64_t a, uint64_t b,
__int128 c) {
return x + y + z + a + c;
}
Gcc generates identical code for these because __int128 have to be stored
aligned on stack (psABI says:
Arguments of type __int128 offer the same operations as INTEGERs, yet they do
not fit into one general purpose register but require two registers.
For classification purposes __int128 is treated as if it were implemented as:
typedef struct {
long low, high;
} __int128;
with the exception that arguments of type __int128 that are stored in memory
must be aligned on a 16-byte boundary.
Clang generates different code for these two functions:
https://godbolt.org/z/ZKjb4Z
Note that both GCC and Clang show the alignment of __int128 properly as 16
bytes, but this is only getting ignored when the variable is passed on the
stack.
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