https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101422
--- Comment #7 from Simon Willcocks <simon.willcocks at gmx dot de> --- (In reply to Jakub Jelinek from comment #6) > Moving the line is the right fix Thanks for the confirmation. > It is recommended to initialize normal automatic variables first and then > only initialize register vars to those automatic variables right before the > inline asm, with no other code in between. In retrospect, I see that that is what this paragraph in https://gcc.gnu.org/onlinedocs/gcc-10.3.0/gcc/Local-Register-Variables.html is trying to say: "Warning: In the above example, be aware that a register (for example r0) can be call-clobbered by subsequent code, including function calls and library calls for arithmetic operators on other variables (for example the initialization of p2). In this case, use temporary variables for expressions between the register assignments:" For a coder to identify if they've encountered "this case" is very difficult (at least, it took me several days), I think perhaps a hard-and-fast rule would be more helpful? (With a change to the example to match.) "Warning: any expression used to initialise a local register variable more complex than a simple assignment from a local variable may involve a compiler-generated function call that could invalidate previously initialised register variables. Therefore, it is recommended that programs only initialize register variables to the values of previously initialised automatic variables just before the inline asm, with no other code in between." int *p1value = …; int *p2value = …; register int *p1 asm ("r0") = p1value; register int *p2 asm ("r1") = p2value; register int *result asm ("r0"); asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2)); (I would still like to see a warning like "local register variable may not be used as expected", but there can't be all that many people using this feature.)
