Thanks for the quick response Richard! Unfortunately the source change suggestion didn't work, but that's OK because I can make the asm work by eliminating some register pressure.
Bottom line is I was looking to find out if there was a compiler problem under the hood. Sounds like the answer is no, or atleast, nothing that's going to be fixed anytime soon. thanks again! Mitch > -----Original Message----- > From: Richard Henderson [mailto:r...@redhat.com] > Sent: Tuesday, November 22, 2011 11:35 AM > To: Bodart, Mitch L > Cc: gcc@gcc.gnu.org > Subject: Re: gcc register allocation error when reloading an asm > > On 11/22/2011 10:55 AM, Bodart, Mitch L wrote: > > What aspect of inline asm processing is preventing gcc from allocating > > register eax to both %0 and %1's address in the following test case? > > > > gcc -m32 -S foo.c > > foo.c: In function 'foo': > > foo.c:20: error: can't find a register in class 'GENERAL_REGS' > while reloading 'asm' > > > > I could understand this error if val was an early clobber, "=&r" > (val), > > as the description of "&" says such operands cannot be assigned to the > > same register as an input, or any register used in a memory address. > > But since no ampersand is present, it seems eax is available here > > for both operands. > > > > int foo(int *ptr) { > > int val; > > > > __asm__ __volatile__("junk %0, %1" > > : "=r" (val), "+m" (*ptr) > > %1 is also an output operand (+), which means its address needs to be > available as an output address reload. This will conflict with normal > output reloads. > > I think I know what you are trying to write: %1 as a RW memory, but > the address is latched on input; %0 as an independent output. > > You could try writing this as > > : "=r"(val), "=X"(*ptr) : "m"(*ptr) > > instead. Untested, so there might be other gotchas... > > > r~
