Messages in this thread Patch in this message | | | Date | Mon, 20 Jul 2020 13:49:25 -0700 | Subject | [PATCH v3 11/11] x86: support i386 with Clang | From | 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@infradead.org/ Link: https://lore.kernel.org/lkml/CAK8P3a1EBaWdbAEzirFDSgHVJMtWjuNt2HGG8z+vpXeNHwETFQ@mail.gmail.com/ Reported-by: Arnd Bergmann <arnd@arndb.de> Reported-by: David Woodhouse <dwmw2@infradead.org> Reported-by: Dmitry Golovin <dima@golovin.in> Reported-by: Linus Torvalds <torvalds@linux-foundation.org> Acked-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> --- 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
| |