On Fri, 2025-06-06 at 16:25 +0800, LIU Hao wrote: > 在 2025-6-5 21:18, Segher Boessenkool 写道: > > Hi! > > > > On Thu, Jun 05, 2025 at 02:14:24PM +0200, Georg-Johann Lay via Gcc-help wrote: > > > Am 05.06.25 um 13:43 schrieb Xi Ruoyao via Gcc-help: > > > > When we write syscall wrappers with inline assembly, we often need to > > > > assign one register for both an input variable and an output variable. > > > > Per https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html, > > > > such an inline assembly can be written > > > > > > > > register int *p1 asm ("r0") = …; > > > > register int *p2 asm ("r1") = …; > > > > register int *result asm ("r0"); > > > > asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2)); > > > > > > > > Note the use of constraint "0" for p1 which shares the same register as > > > > result. But currently in the Linux kernel vDSO, they are actually > > > > written like > > > > > > > > asm ("sysint" : "=r" (result) : "r" (p1), "r" (p2)); > > > > > > > > So the question here: is using "r" for p1 valid or not? Do we need to > > > > "fix" this everywhere for the vDSO? > > > > > > That code is perfectly fine, since the register numbers for p1 and > > > result are the same "r0" at the point of the inline asm. > > > > Yup. Various compiler passes will not realise "result" is the same reg > > as "p1", but that probably won't limit optimisation here. > > FWIW, maybe that's too much into the implementation details of GCC. Clang does not treat `p1` and > `result` as referencing the same datum; a value that is written into one of them is not visible from the > other, which can produce strange results like https://gcc.godbolt.org/z/nohrc9oxq > > So the suggestion below is much much better. > > (And this asm statement is likely a candidate for `volatile`.) Yes, in the real kernel code they are volatile, and with volatile Clang does the correct thing. But Clang even refuse to compile it: inline int rr_add_store(int* p) { register int t __asm__("eax"); register int old __asm__("eax") = *p; __asm__ volatile ("nop" : "+r"(t) : "0"(old)); t += 100; *p = t; return old; } results in <source>:7:41: error: invalid input constraint '0' in asm 7 | __asm__ volatile ("nop" : "+r"(t) : "0"(old)); | ^ -- Xi Ruoyao <xry111@xxxxxxxxxxx> School of Aerospace Science and Technology, Xidian University
