kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 107 insertions, 0 deletions

diff --git a/include/math-emu/op-8.h b/include/math-emu/op-8.h
new file mode 100644
index 000000000..8b8c05eb8
--- /dev/null
+++ b/include/math-emu/op-8.h
@@ -0,0 +1,107 @@
+/* Software floating-point emulation.
+   Basic eight-word fraction declaration and manipulation.
+   Copyright (C) 1997,1998,1999 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by Richard Henderson (rth@cygnus.com),
+		  Jakub Jelinek (jj@ultra.linux.cz) and
+		  Peter Maydell (pmaydell@chiark.greenend.org.uk).
+                                                         
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Library General Public License as
+   published by the Free Software Foundation; either version 2 of the
+   License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Library General Public License for more details.
+
+   You should have received a copy of the GNU Library General Public
+   License along with the GNU C Library; see the file COPYING.LIB.  If
+   not, write to the Free Software Foundation, Inc.,
+   59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */
+
+#ifndef __MATH_EMU_OP_8_H__
+#define __MATH_EMU_OP_8_H__
+
+/* We need just a few things from here for op-4, if we ever need some
+   other macros, they can be added. */
+#define _FP_FRAC_DECL_8(X)	_FP_W_TYPE X##_f[8]
+#define _FP_FRAC_HIGH_8(X)	(X##_f[7])
+#define _FP_FRAC_LOW_8(X)	(X##_f[0])
+#define _FP_FRAC_WORD_8(X,w)	(X##_f[w])
+
+#define _FP_FRAC_SLL_8(X,N)						\
+  do {									\
+    _FP_I_TYPE _up, _down, _skip, _i;					\
+    _skip = (N) / _FP_W_TYPE_SIZE;					\
+    _up = (N) % _FP_W_TYPE_SIZE;					\
+    _down = _FP_W_TYPE_SIZE - _up;					\
+    if (!_up)								\
+      for (_i = 7; _i >= _skip; --_i)					\
+	X##_f[_i] = X##_f[_i-_skip];					\
+    else								\
+      {									\
+	for (_i = 7; _i > _skip; --_i)					\
+	  X##_f[_i] = X##_f[_i-_skip] << _up				\
+		      | X##_f[_i-_skip-1] >> _down;			\
+	X##_f[_i--] = X##_f[0] << _up; 					\
+      }									\
+    for (; _i >= 0; --_i)						\
+      X##_f[_i] = 0;							\
+  } while (0)
+
+#define _FP_FRAC_SRL_8(X,N)						\
+  do {									\
+    _FP_I_TYPE _up, _down, _skip, _i;					\
+    _skip = (N) / _FP_W_TYPE_SIZE;					\
+    _down = (N) % _FP_W_TYPE_SIZE;					\
+    _up = _FP_W_TYPE_SIZE - _down;					\
+    if (!_down)								\
+      for (_i = 0; _i <= 7-_skip; ++_i)					\
+	X##_f[_i] = X##_f[_i+_skip];					\
+    else								\
+      {									\
+	for (_i = 0; _i < 7-_skip; ++_i)				\
+	  X##_f[_i] = X##_f[_i+_skip] >> _down				\
+		      | X##_f[_i+_skip+1] << _up;			\
+	X##_f[_i++] = X##_f[7] >> _down;				\
+      }									\
+    for (; _i < 8; ++_i)						\
+      X##_f[_i] = 0;							\
+  } while (0)
+
+
+/* Right shift with sticky-lsb. 
+ * What this actually means is that we do a standard right-shift,
+ * but that if any of the bits that fall off the right hand side
+ * were one then we always set the LSbit.
+ */
+#define _FP_FRAC_SRS_8(X,N,size)					\
+  do {									\
+    _FP_I_TYPE _up, _down, _skip, _i;					\
+    _FP_W_TYPE _s;							\
+    _skip = (N) / _FP_W_TYPE_SIZE;					\
+    _down = (N) % _FP_W_TYPE_SIZE;					\
+    _up = _FP_W_TYPE_SIZE - _down;					\
+    for (_s = _i = 0; _i < _skip; ++_i)					\
+      _s |= X##_f[_i];							\
+    _s |= X##_f[_i] << _up;						\
+/* s is now != 0 if we want to set the LSbit */				\
+    if (!_down)								\
+      for (_i = 0; _i <= 7-_skip; ++_i)					\
+	X##_f[_i] = X##_f[_i+_skip];					\
+    else								\
+      {									\
+	for (_i = 0; _i < 7-_skip; ++_i)				\
+	  X##_f[_i] = X##_f[_i+_skip] >> _down				\
+		      | X##_f[_i+_skip+1] << _up;			\
+	X##_f[_i++] = X##_f[7] >> _down;				\
+      }									\
+    for (; _i < 8; ++_i)						\
+      X##_f[_i] = 0;							\
+    /* don't fix the LSB until the very end when we're sure f[0] is stable */	\
+    X##_f[0] |= (_s != 0);						\
+  } while (0)
+
+#endif