[PATCH v3 11/11] x86: support i386 with Clang

[Nick Desaulniers]

GCC and Clang are architecturally different, which leads to subtle
issues for code that's invalid but clearly dead. This can happen with
code that emulates polymorphism with the preprocessor and sizeof.

GCC will perform semantic analysis after early inlining and dead code
elimination, so it will not warn on invalid code that's dead. Clang
strictly performs optimizations after semantic analysis, so it will warn
for dead code.

Neither Clang nor GCC like this very much with -m32:

long long ret;
asm ("movb $5, %0" : "=q" (ret));

However, GCC can tolerate this variant:

long long ret;
switch (sizeof(ret)) {
case 1:
        asm ("movb $5, %0" : "=q" (ret));
        break;
case 8:;
}

Clang, on the other hand, won't accept that because it validates the
inline asm for the '1' case *before* the optimisation phase where it
realises that it wouldn't have to emit it anyway.

If LLVM (Clang's "back end") fails such as during instruction selection
or register allocation, it cannot provide accurate diagnostics
(warnings/errors) that contain line information, as the AST has been
discarded from memory at that point.

While there have been early discussions about having C/C++ specific
language optimizations in Clang via the use of MLIR, which would enable
such earlier optimizations, such work is not scoped and likely a
multi-year endeavor.

We also don't want to swap the use of "=q" with "=r". For 64b, it
doesn't matter. For 32b, it's possible that a 32b register without a 8b
lower alias (i.e. ESI, EDI, EBP) is selected which the assembler will
then reject.

With this, Clang can finally build an i386 defconfig.

Link: https://bugs.llvm.org/show_bug.cgi?id=33587
Link: https://github.com/ClangBuiltLinux/linux/issues/3
Link: https://github.com/ClangBuiltLinux/linux/issues/194
Link: https://github.com/ClangBuiltLinux/linux/issues/781
Link: https://lore.kernel.org/lkml/20180209161833.4605-1-dwmw2@xxxxxxxxxxxxx/
Link: https://lore.kernel.org/lkml/CAK8P3a1EBaWdbAEzirFDSgHVJMtWjuNt2HGG8z+vpXeNHwETFQ@xxxxxxxxxxxxxx/
Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
Reported-by: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Reported-by: Dmitry Golovin <dima@xxxxxxxxxx>
Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
 arch/x86/include/asm/uaccess.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index dd3261f9f4ea..9d57556ad42f 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -314,11 +314,13 @@ do {									\
 
 #define __get_user_size(x, ptr, size, retval)				\
 do {									\
+	unsigned char x_u8__;						\
 	retval = 0;							\
 	__chk_user_ptr(ptr);						\
 	switch (size) {							\
 	case 1:								\
-		__get_user_asm(x, ptr, retval, "b", "=q");		\
+		__get_user_asm(x_u8__, ptr, retval, "b", "=q");		\
+		(x) = x_u8__;						\
 		break;							\
 	case 2:								\
 		__get_user_asm(x, ptr, retval, "w", "=r");		\
-- 
2.28.0.rc0.105.gf9edc3c819-goog