powerpc: Work around inline asm issues in alternate feature sections

The clang toolchain treats inline assembly a bit differently than
straight assembly code. In particular, inline assembly doesn't have
the complete context available to resolve expressions. This is
intentional to avoid divergence in the resulting assembly code.

We can work around this issue by borrowing a workaround done for ARM,
i.e. not directly testing the labels themselves, but by moving the
current output pointer by a value that should always be zero. If this
value is not null, then we will trigger a backward move, which is
explicitly forbidden.

Signed-off-by: Bill Wendling <morbo@google.com>
[mpe: Put it in a macro and only do the workaround for clang]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201120224034.191382-4-morbo@google.com

arch/powerpc/include/asm/feature-fixups.h

@@ -36,6 +36,24 @@ label##2:						\
 	.align 2;					\
 label##3:
 
+
+#ifndef CONFIG_CC_IS_CLANG
+#define CHECK_ALT_SIZE(else_size, body_size)			\
+	.ifgt (else_size) - (body_size);			\
+	.error "Feature section else case larger than body";	\
+	.endif;
+#else
+/*
+ * If we use the ifgt syntax above, clang's assembler complains about the
+ * expression being non-absolute when the code appears in an inline assembly
+ * statement.
+ * As a workaround use an .org directive that has no effect if the else case
+ * instructions are smaller than the body, but fails otherwise.
+ */
+#define CHECK_ALT_SIZE(else_size, body_size)			\
+	.org . + ((else_size) > (body_size));
+#endif
+
 #define MAKE_FTR_SECTION_ENTRY(msk, val, label, sect)		\
 label##4:							\
 	.popsection;						\
@@ -48,9 +66,7 @@ label##5:							\
 	FTR_ENTRY_OFFSET label##2b-label##5b;			\
 	FTR_ENTRY_OFFSET label##3b-label##5b;			\
 	FTR_ENTRY_OFFSET label##4b-label##5b;			\
-	.ifgt (label##4b- label##3b)-(label##2b- label##1b);	\
+	CHECK_ALT_SIZE((label##4b-label##3b), (label##2b-label##1b)); \
-	.error "Feature section else case larger than body";	\
-	.endif;							\
 	.popsection;