On Wed, Oct 13, 2021 at 7:44 PM Michael Ellerman <m...@ellerman.id.au> wrote: > > In dcr-low.S we use cmpli with three arguments, instead of four > arguments as defined in the ISA: > > cmpli cr0,r3,1024 > > This appears to be a PPC440-ism, looking at the "PPC440x5 CPU Core > User’s Manual" it shows cmpli having no L field, but implied to be 0 due > to the core being 32-bit. It mentions that the ISA defines four > arguments and recommends using cmplwi. > > dcr-low.S is only built 32-bit, because it is only built when > DCR_NATIVE=y, which is only selected by 40x and 44x. Looking at the > generated code (with gcc/gas) we see cmplwi as expected. > > Although gas is happy with the 3-argument version when building for > 32-bit, the LLVM assembler is not and errors out with: > > arch/powerpc/sysdev/dcr-low.S:27:10: error: invalid operand for instruction > cmpli 0,%r3,1024; ... > ^ > > Switching to the four argument version avoids any confusion when reading > the ISA, fixes the issue with the LLVM assembler, and also means the > code could be built 64-bit in future (though that's very unlikely).
Thank you Michael. We've definitely run into a few cases where GAS allowed for various short-hand forms of various instructions (a fair amount of recent work was around 32b ARM and THUMB parity in LLVM). LLVM's assembler is mostly generated from a high level description of the instruction formats, so it's not always as flexible as a hand written parser would be. (There is a mix of hand written arch specific parsing, but most of the parser is arch agnostic, and all of the instruction descriptions are described in an LLVM specific high level language called tablegen which generates C++ that is used by the assembler, but also the disassembler, the compiler, and even the linker if need be). Link: https://github.com/ClangBuiltLinux/linux/issues/1419 Reviewed-by: Nick Desaulniers <ndesaulni...@google.com> > Reported-by: Nick Desaulniers <ndesaulni...@google.com> > Signed-off-by: Michael Ellerman <m...@ellerman.id.au> > --- > arch/powerpc/sysdev/dcr-low.S | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/powerpc/sysdev/dcr-low.S b/arch/powerpc/sysdev/dcr-low.S > index efeeb1b885a1..329b9c4ae542 100644 > --- a/arch/powerpc/sysdev/dcr-low.S > +++ b/arch/powerpc/sysdev/dcr-low.S > @@ -11,7 +11,7 @@ > #include <asm/export.h> > > #define DCR_ACCESS_PROLOG(table) \ > - cmpli cr0,r3,1024; \ > + cmplwi cr0,r3,1024; \ > rlwinm r3,r3,4,18,27; \ > lis r5,table@h; \ > ori r5,r5,table@l; \ > -- > 2.25.1 > -- Thanks, ~Nick Desaulniers
