powerpc/dcr: Use cmplwi instead of 3-argument cmpli

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.

It also corresponds to the old POWER instruction set, which had no L
field there, a reserved bit instead.

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; ...
           ^

Switch to the cmplwi extended opcode, which 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).

Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
BugLink: https://github.com/ClangBuiltLinux/linux/issues/1419
Link: https://lore.kernel.org/r/20211014024424.528848-1-mpe@ellerman.id.au

diff --git a/arch/powerpc/sysdev/dcr-low.S b/arch/powerpc/sysdev/dcr-low.S
index efeeb1b..329b9c4 100644 (file)
--- 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;   \