https://bugs.llvm.org/show_bug.cgi?id=47447
Bug ID: 47447
Summary: bad and questionable codegen for inline asm returning
a value in a fixed register
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Backend: RISC-V
Assignee: unassignedb...@nondot.org
Reporter: sor...@fastmail.com
CC: a...@lowrisc.org, llvm-bugs@lists.llvm.org
__builtin_thread_pointer() is not supported on released versions of risc-v gcc
or clang, so I looked for alternatives that would generate the same code. I
tried:
void *get_tp() {
register void *tp asm("tp");
asm("" : "=r"(tp));
return tp;
}
define i8* @get_tp() {
%1 = tail call i8* asm "", "={x4}"()
ret i8* %1
}
(https://gcc.godbolt.org/z/GP5ab4)
This works fine on released gcc (can even fold into subsequent memory accesses
which become tp-relative) but llvm generates a superfluous stack frame as
though tp were a call-saved register. Since tp is a fixed register, I don't
think llvm is _allowed_ to insert saves and restores for it; the asm might be
erroneous, I am not sure of the intended semantics, but if it is accepted it
should be accepted without generating a stack frame.
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