[PATCH] 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.

Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
Reported-by: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Reported-by: Dmitry Golovin <dima@xxxxxxxxxx>
Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
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/
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
Note: this is a resend of:
https://lore.kernel.org/lkml/20180209161833.4605-1-dwmw2@xxxxxxxxxxxxx/
rebased on today's Linux next, and with the additional change to
uaccess.h.

I'm happy to resend with authorship and reported by tags changed to
suggested by's or whatever, just let me know.

Part of the commit message is stolen from David, and part from Linus.
Shall I resend with David's authorship and
[Nick: reworded]
???

I don't really care, I just don't really want to carry this out of tree
for our CI, which is green for i386 otherwise.


 arch/x86/include/asm/percpu.h  | 12 ++++++++----
 arch/x86/include/asm/uaccess.h |  4 +++-
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 2278797c769d..826d086f71c9 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -99,7 +99,7 @@ do {							\
 	case 1:						\
 		asm qual (op "b %1,"__percpu_arg(0)	\
 		    : "+m" (var)			\
-		    : "qi" ((pto_T__)(val)));		\
+		    : "qi" ((unsigned char)(unsigned long)(val))); \
 		break;					\
 	case 2:						\
 		asm qual (op "w %1,"__percpu_arg(0)	\
@@ -144,7 +144,7 @@ do {									\
 		else							\
 			asm qual ("addb %1, "__percpu_arg(0)		\
 			    : "+m" (var)				\
-			    : "qi" ((pao_T__)(val)));			\
+			    : "qi" ((unsigned char)(unsigned long)(val))); \
 		break;							\
 	case 2:								\
 		if (pao_ID__ == 1)					\
@@ -182,12 +182,14 @@ do {									\
 
 #define percpu_from_op(qual, op, var)			\
 ({							\
+	unsigned char pfo_u8__;				\
 	typeof(var) pfo_ret__;				\
 	switch (sizeof(var)) {				\
 	case 1:						\
 		asm qual (op "b "__percpu_arg(1)",%0"	\
-		    : "=q" (pfo_ret__)			\
+		    : "=q" (pfo_u8__)			\
 		    : "m" (var));			\
+		pfo_ret__ = (typeof(var))(unsigned long)pfo_u8__;	\
 		break;					\
 	case 2:						\
 		asm qual (op "w "__percpu_arg(1)",%0"	\
@@ -211,12 +213,14 @@ do {									\
 
 #define percpu_stable_op(op, var)			\
 ({							\
+	unsigned char pfo_u8__;				\
 	typeof(var) pfo_ret__;				\
 	switch (sizeof(var)) {				\
 	case 1:						\
 		asm(op "b "__percpu_arg(P1)",%0"	\
-		    : "=q" (pfo_ret__)			\
+		    : "=q" (pfo_u8__)			\
 		    : "p" (&(var)));			\
+		pfo_ret__ = (typeof(var))(unsigned long)pfo_u8__;	\
 		break;					\
 	case 2:						\
 		asm(op "w "__percpu_arg(P1)",%0"	\
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index d8f283b9a569..cf8483cd80e1 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");		\
-- 
2.26.2.526.g744177e7f7-goog