On 23.02.2024 14:03, Roger Pau Monne wrote:
> The current code for alternative calls uses the caller parameter types as the
> types for the register variables that serve as function parameters:
>
> uint8_t foo;
> [...]
> alternative_call(myfunc, foo);
>
> Would expand roughly into:
>
> register unint8_t a1_ asm("rdi") = foo;
> register unsigned long a2_ asm("rsi");
> [...]
> asm volatile ("call *%c[addr](%%rip)"...);
>
> However with -O2 clang will generate incorrect code, given the following
> example:
>
> unsigned int func(uint8_t t)
> {
> return t;
> }
>
> static void bar(uint8_t b)
> {
> int ret_;
> register uint8_t di asm("rdi") = b;
> register unsigned long si asm("rsi");
> register unsigned long dx asm("rdx");
> register unsigned long cx asm("rcx");
> register unsigned long r8 asm("r8");
> register unsigned long r9 asm("r9");
> register unsigned long r10 asm("r10");
> register unsigned long r11 asm("r11");
>
> asm volatile ( "call %c[addr]"
> : "+r" (di), "=r" (si), "=r" (dx),
> "=r" (cx), "=r" (r8), "=r" (r9),
> "=r" (r10), "=r" (r11), "=a" (ret_)
> : [addr] "i" (&(func)), "g" (func)
> : "memory" );
> }
>
> void foo(unsigned int a)
> {
> bar(a);
> }
>
> Clang generates the following assembly code:
>
> func: # @func
> movl %edi, %eax
> retq
> foo: # @foo
> callq func
> retq
>
> Note the truncation of the unsigned int parameter 'a' of foo() to uint8_t when
> passed into bar() is lost. clang doesn't zero extend the parameters in the
> callee when required, as the psABI mandates.
>
> The above can be worked around by using an union when defining the register
> variables, so that `di` becomes:
>
> register union {
> uint8_t e;
> unsigned long r;
> } di asm("rdi") = { .e = b };
>
> Which results in following code generated for `foo()`:
>
> foo: # @foo
> movzbl %dil, %edi
> callq func
> retq
>
> So the truncation is not longer lost. Apply such workaround only when built
> with clang.
>
> Reported-by: Matthew Grooms <mgro...@shrew.net>
> Link: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=277200
> Link: https://github.com/llvm/llvm-project/issues/12579
> Link: https://github.com/llvm/llvm-project/issues/82598
> Signed-off-by: Roger Pau Monné <roger....@citrix.com>
Acked-by: Jan Beulich <jbeul...@suse.com>
in the hopes that this isn't going to bite us later.
Jan
