clang: work around asm input constraint problems

Work around clang problems with asm constraints that have multiple
possibilities, particularly "g" and "rm".

Clang seems to turn inputs like that into the most generic form, which
is the memory input - but to make matters worse, clang won't even use a
possible original memory location, but will spill the value to stack,
and use the stack for the asm input.

See

  https://github.com/llvm/llvm-project/issues/20571#issuecomment-980933442

for some explanation of why clang has this strange behavior, but the end
result is that "g" and "rm" really end up generating horrid code.

Link: https://github.com/llvm/llvm-project/issues/20571
Cc: Peter Zijlstra <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 10 insertions, 0 deletions

diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index 49feac0162a5..4c1a39dcb624 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -118,3 +118,13 @@
 
 #define __diag_ignore_all(option, comment) \
 	__diag_clang(13, ignore, option)
+
+/*
+ * clang has horrible behavior with "g" or "rm" constraints for asm
+ * inputs, turning them into something worse than "m". Avoid using
+ * constraints with multiple possible uses (but "ir" seems to be ok):
+ *
+ *	https://github.com/llvm/llvm-project/issues/20571
+ */
+#define ASM_INPUT_G "ir"
+#define ASM_INPUT_RM "r"