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, 205 insertions, 0 deletions

diff --git a/include/math-emu/double.h b/include/math-emu/double.h
new file mode 100644
index 000000000..655ccf1c4
--- /dev/null
+++ b/include/math-emu/double.h
@@ -0,0 +1,205 @@
+/* Software floating-point emulation.
+   Definitions for IEEE Double Precision
+   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),
+		  David S. Miller (davem@redhat.com) 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_DOUBLE_H__
+#define    __MATH_EMU_DOUBLE_H__
+
+#if _FP_W_TYPE_SIZE < 32
+#error "Here's a nickel kid.  Go buy yourself a real computer."
+#endif
+
+#if _FP_W_TYPE_SIZE < 64
+#define _FP_FRACTBITS_D		(2 * _FP_W_TYPE_SIZE)
+#else
+#define _FP_FRACTBITS_D		_FP_W_TYPE_SIZE
+#endif
+
+#define _FP_FRACBITS_D		53
+#define _FP_FRACXBITS_D		(_FP_FRACTBITS_D - _FP_FRACBITS_D)
+#define _FP_WFRACBITS_D		(_FP_WORKBITS + _FP_FRACBITS_D)
+#define _FP_WFRACXBITS_D	(_FP_FRACTBITS_D - _FP_WFRACBITS_D)
+#define _FP_EXPBITS_D		11
+#define _FP_EXPBIAS_D		1023
+#define _FP_EXPMAX_D		2047
+
+#define _FP_QNANBIT_D		\
+	((_FP_W_TYPE)1 << (_FP_FRACBITS_D-2) % _FP_W_TYPE_SIZE)
+#define _FP_IMPLBIT_D		\
+	((_FP_W_TYPE)1 << (_FP_FRACBITS_D-1) % _FP_W_TYPE_SIZE)
+#define _FP_OVERFLOW_D		\
+	((_FP_W_TYPE)1 << _FP_WFRACBITS_D % _FP_W_TYPE_SIZE)
+
+#if _FP_W_TYPE_SIZE < 64
+
+union _FP_UNION_D
+{
+  double flt;
+  struct {
+#if __BYTE_ORDER == __BIG_ENDIAN
+    unsigned sign  : 1;
+    unsigned exp   : _FP_EXPBITS_D;
+    unsigned frac1 : _FP_FRACBITS_D - (_FP_IMPLBIT_D != 0) - _FP_W_TYPE_SIZE;
+    unsigned frac0 : _FP_W_TYPE_SIZE;
+#else
+    unsigned frac0 : _FP_W_TYPE_SIZE;
+    unsigned frac1 : _FP_FRACBITS_D - (_FP_IMPLBIT_D != 0) - _FP_W_TYPE_SIZE;
+    unsigned exp   : _FP_EXPBITS_D;
+    unsigned sign  : 1;
+#endif
+  } bits __attribute__((packed));
+};
+
+#define FP_DECL_D(X)		_FP_DECL(2,X)
+#define FP_UNPACK_RAW_D(X,val)	_FP_UNPACK_RAW_2(D,X,val)
+#define FP_UNPACK_RAW_DP(X,val)	_FP_UNPACK_RAW_2_P(D,X,val)
+#define FP_PACK_RAW_D(val,X)	_FP_PACK_RAW_2(D,val,X)
+#define FP_PACK_RAW_DP(val,X)		\
+  do {					\
+    if (!FP_INHIBIT_RESULTS)		\
+      _FP_PACK_RAW_2_P(D,val,X);	\
+  } while (0)
+
+#define FP_UNPACK_D(X,val)		\
+  do {					\
+    _FP_UNPACK_RAW_2(D,X,val);		\
+    _FP_UNPACK_CANONICAL(D,2,X);	\
+  } while (0)
+
+#define FP_UNPACK_DP(X,val)		\
+  do {					\
+    _FP_UNPACK_RAW_2_P(D,X,val);	\
+    _FP_UNPACK_CANONICAL(D,2,X);	\
+  } while (0)
+
+#define FP_PACK_D(val,X)		\
+  do {					\
+    _FP_PACK_CANONICAL(D,2,X);		\
+    _FP_PACK_RAW_2(D,val,X);		\
+  } while (0)
+
+#define FP_PACK_DP(val,X)		\
+  do {					\
+    _FP_PACK_CANONICAL(D,2,X);		\
+    if (!FP_INHIBIT_RESULTS)		\
+      _FP_PACK_RAW_2_P(D,val,X);	\
+  } while (0)
+
+#define FP_ISSIGNAN_D(X)		_FP_ISSIGNAN(D,2,X)
+#define FP_NEG_D(R,X)			_FP_NEG(D,2,R,X)
+#define FP_ADD_D(R,X,Y)			_FP_ADD(D,2,R,X,Y)
+#define FP_SUB_D(R,X,Y)			_FP_SUB(D,2,R,X,Y)
+#define FP_MUL_D(R,X,Y)			_FP_MUL(D,2,R,X,Y)
+#define FP_DIV_D(R,X,Y)			_FP_DIV(D,2,R,X,Y)
+#define FP_SQRT_D(R,X)			_FP_SQRT(D,2,R,X)
+#define _FP_SQRT_MEAT_D(R,S,T,X,Q)	_FP_SQRT_MEAT_2(R,S,T,X,Q)
+
+#define FP_CMP_D(r,X,Y,un)	_FP_CMP(D,2,r,X,Y,un)
+#define FP_CMP_EQ_D(r,X,Y)	_FP_CMP_EQ(D,2,r,X,Y)
+
+#define FP_TO_INT_D(r,X,rsz,rsg)	_FP_TO_INT(D,2,r,X,rsz,rsg)
+#define FP_TO_INT_ROUND_D(r,X,rsz,rsg)	_FP_TO_INT_ROUND(D,2,r,X,rsz,rsg)
+#define FP_FROM_INT_D(X,r,rs,rt)	_FP_FROM_INT(D,2,X,r,rs,rt)
+
+#define _FP_FRAC_HIGH_D(X)	_FP_FRAC_HIGH_2(X)
+#define _FP_FRAC_HIGH_RAW_D(X)	_FP_FRAC_HIGH_2(X)
+
+#else
+
+union _FP_UNION_D
+{
+  double flt;
+  struct {
+#if __BYTE_ORDER == __BIG_ENDIAN
+    unsigned sign : 1;
+    unsigned exp  : _FP_EXPBITS_D;
+    unsigned long frac : _FP_FRACBITS_D - (_FP_IMPLBIT_D != 0);
+#else
+    unsigned long frac : _FP_FRACBITS_D - (_FP_IMPLBIT_D != 0);
+    unsigned exp  : _FP_EXPBITS_D;
+    unsigned sign : 1;
+#endif
+  } bits __attribute__((packed));
+};
+
+#define FP_DECL_D(X)		_FP_DECL(1,X)
+#define FP_UNPACK_RAW_D(X,val)	_FP_UNPACK_RAW_1(D,X,val)
+#define FP_UNPACK_RAW_DP(X,val)	_FP_UNPACK_RAW_1_P(D,X,val)
+#define FP_PACK_RAW_D(val,X)	_FP_PACK_RAW_1(D,val,X)
+#define FP_PACK_RAW_DP(val,X)		\
+  do {					\
+    if (!FP_INHIBIT_RESULTS)		\
+      _FP_PACK_RAW_1_P(D,val,X);	\
+  } while (0)
+
+#define FP_UNPACK_D(X,val)		\
+  do {					\
+    _FP_UNPACK_RAW_1(D,X,val);		\
+    _FP_UNPACK_CANONICAL(D,1,X);	\
+  } while (0)
+
+#define FP_UNPACK_DP(X,val)		\
+  do {					\
+    _FP_UNPACK_RAW_1_P(D,X,val);	\
+    _FP_UNPACK_CANONICAL(D,1,X);	\
+  } while (0)
+
+#define FP_PACK_D(val,X)		\
+  do {					\
+    _FP_PACK_CANONICAL(D,1,X);		\
+    _FP_PACK_RAW_1(D,val,X);		\
+  } while (0)
+
+#define FP_PACK_DP(val,X)		\
+  do {					\
+    _FP_PACK_CANONICAL(D,1,X);		\
+    if (!FP_INHIBIT_RESULTS)		\
+      _FP_PACK_RAW_1_P(D,val,X);	\
+  } while (0)
+
+#define FP_ISSIGNAN_D(X)		_FP_ISSIGNAN(D,1,X)
+#define FP_NEG_D(R,X)			_FP_NEG(D,1,R,X)
+#define FP_ADD_D(R,X,Y)			_FP_ADD(D,1,R,X,Y)
+#define FP_SUB_D(R,X,Y)			_FP_SUB(D,1,R,X,Y)
+#define FP_MUL_D(R,X,Y)			_FP_MUL(D,1,R,X,Y)
+#define FP_DIV_D(R,X,Y)			_FP_DIV(D,1,R,X,Y)
+#define FP_SQRT_D(R,X)			_FP_SQRT(D,1,R,X)
+#define _FP_SQRT_MEAT_D(R,S,T,X,Q)	_FP_SQRT_MEAT_1(R,S,T,X,Q)
+
+/* The implementation of _FP_MUL_D and _FP_DIV_D should be chosen by
+   the target machine.  */
+
+#define FP_CMP_D(r,X,Y,un)	_FP_CMP(D,1,r,X,Y,un)
+#define FP_CMP_EQ_D(r,X,Y)	_FP_CMP_EQ(D,1,r,X,Y)
+
+#define FP_TO_INT_D(r,X,rsz,rsg)	_FP_TO_INT(D,1,r,X,rsz,rsg)
+#define FP_TO_INT_ROUND_D(r,X,rsz,rsg)	_FP_TO_INT_ROUND(D,1,r,X,rsz,rsg)
+#define FP_FROM_INT_D(X,r,rs,rt)	_FP_FROM_INT(D,1,X,r,rs,rt)
+
+#define _FP_FRAC_HIGH_D(X)	_FP_FRAC_HIGH_1(X)
+#define _FP_FRAC_HIGH_RAW_D(X)	_FP_FRAC_HIGH_1(X)
+
+#endif /* W_TYPE_SIZE < 64 */
+
+
+#endif /* __MATH_EMU_DOUBLE_H__ */