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, 263 insertions, 0 deletions

diff --git a/arch/alpha/lib/ev6-divide.S b/arch/alpha/lib/ev6-divide.S
new file mode 100644
index 000000000..b73a6d263
--- /dev/null
+++ b/arch/alpha/lib/ev6-divide.S
@@ -0,0 +1,263 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * arch/alpha/lib/ev6-divide.S
+ *
+ * 21264 version contributed by Rick Gorton <rick.gorton@alpha-processor.com>
+ *
+ * Alpha division..
+ */
+
+/*
+ * The alpha chip doesn't provide hardware division, so we have to do it
+ * by hand.  The compiler expects the functions
+ *
+ *	__divqu: 64-bit unsigned long divide
+ *	__remqu: 64-bit unsigned long remainder
+ *	__divqs/__remqs: signed 64-bit
+ *	__divlu/__remlu: unsigned 32-bit
+ *	__divls/__remls: signed 32-bit
+ *
+ * These are not normal C functions: instead of the normal
+ * calling sequence, these expect their arguments in registers
+ * $24 and $25, and return the result in $27. Register $28 may
+ * be clobbered (assembly temporary), anything else must be saved. 
+ *
+ * In short: painful.
+ *
+ * This is a rather simple bit-at-a-time algorithm: it's very good
+ * at dividing random 64-bit numbers, but the more usual case where
+ * the divisor is small is handled better by the DEC algorithm
+ * using lookup tables. This uses much less memory, though, and is
+ * nicer on the cache.. Besides, I don't know the copyright status
+ * of the DEC code.
+ */
+
+/*
+ * My temporaries:
+ *	$0 - current bit
+ *	$1 - shifted divisor
+ *	$2 - modulus/quotient
+ *
+ *	$23 - return address
+ *	$24 - dividend
+ *	$25 - divisor
+ *
+ *	$27 - quotient/modulus
+ *	$28 - compare status
+ *
+ * Much of the information about 21264 scheduling/coding comes from:
+ *	Compiler Writer's Guide for the Alpha 21264
+ *	abbreviated as 'CWG' in other comments here
+ *	ftp.digital.com/pub/Digital/info/semiconductor/literature/dsc-library.html
+ * Scheduling notation:
+ *	E	- either cluster
+ *	U	- upper subcluster; U0 - subcluster U0; U1 - subcluster U1
+ *	L	- lower subcluster; L0 - subcluster L0; L1 - subcluster L1
+ * Try not to change the actual algorithm if possible for consistency.
+ */
+
+#include <linux/export.h>
+#define halt .long 0
+
+/*
+ * Select function type and registers
+ */
+#define mask	$0
+#define divisor	$1
+#define compare $28
+#define tmp1	$3
+#define tmp2	$4
+
+#ifdef DIV
+#define DIV_ONLY(x,y...) x,##y
+#define MOD_ONLY(x,y...)
+#define func(x) __div##x
+#define modulus $2
+#define quotient $27
+#define GETSIGN(x) xor $24,$25,x
+#define STACK 48
+#else
+#define DIV_ONLY(x,y...)
+#define MOD_ONLY(x,y...) x,##y
+#define func(x) __rem##x
+#define modulus $27
+#define quotient $2
+#define GETSIGN(x) bis $24,$24,x
+#define STACK 32
+#endif
+
+/*
+ * For 32-bit operations, we need to extend to 64-bit
+ */
+#ifdef INTSIZE
+#define ufunction func(lu)
+#define sfunction func(l)
+#define LONGIFY(x) zapnot x,15,x
+#define SLONGIFY(x) addl x,0,x
+#else
+#define ufunction func(qu)
+#define sfunction func(q)
+#define LONGIFY(x)
+#define SLONGIFY(x)
+#endif
+
+.set noat
+.align	4
+.globl	ufunction
+.ent	ufunction
+ufunction:
+	subq	$30,STACK,$30		# E :
+	.frame	$30,STACK,$23
+	.prologue 0
+
+7:	stq	$1, 0($30)		# L :
+	bis	$25,$25,divisor		# E :
+	stq	$2, 8($30)		# L : L U L U
+
+	bis	$24,$24,modulus		# E :
+	stq	$0,16($30)		# L :
+	bis	$31,$31,quotient	# E :
+	LONGIFY(divisor)		# E : U L L U
+
+	stq	tmp1,24($30)		# L :
+	LONGIFY(modulus)		# E :
+	bis	$31,1,mask		# E :
+	DIV_ONLY(stq tmp2,32($30))	# L : L U U L
+
+	beq	divisor, 9f			/* div by zero */
+	/*
+	 * In spite of the DIV_ONLY being either a non-instruction
+	 * or an actual stq, the addition of the .align directive
+	 * below ensures that label 1 is going to be nicely aligned
+	 */
+
+	.align	4
+#ifdef INTSIZE
+	/*
+	 * shift divisor left, using 3-bit shifts for
+	 * 32-bit divides as we can't overflow. Three-bit
+	 * shifts will result in looping three times less
+	 * here, but can result in two loops more later.
+	 * Thus using a large shift isn't worth it (and
+	 * s8add pairs better than a sll..)
+	 */
+1:	cmpult	divisor,modulus,compare	# E :
+	s8addq	divisor,$31,divisor	# E :
+	s8addq	mask,$31,mask		# E :
+	bne	compare,1b		# U : U L U L
+#else
+1:	cmpult	divisor,modulus,compare	# E :
+	nop				# E :
+	nop				# E :
+	blt     divisor, 2f		# U : U L U L
+
+	addq	divisor,divisor,divisor	# E :
+	addq	mask,mask,mask		# E :
+	unop				# E :
+	bne	compare,1b		# U : U L U L
+#endif
+
+	/* ok, start to go right again.. */
+2:
+	/*
+	 * Keep things nicely bundled... use a nop instead of not
+	 * having an instruction for DIV_ONLY
+	 */
+#ifdef DIV
+	DIV_ONLY(addq quotient,mask,tmp2) # E :
+#else
+	nop				# E :
+#endif
+	srl	mask,1,mask		# U :
+	cmpule	divisor,modulus,compare	# E :
+	subq	modulus,divisor,tmp1	# E :
+
+#ifdef DIV
+	DIV_ONLY(cmovne compare,tmp2,quotient)	# E : Latency 2, extra map slot
+	nop				# E : as part of the cmovne
+	srl	divisor,1,divisor	# U :
+	nop				# E : L U L U
+
+	nop				# E :
+	cmovne	compare,tmp1,modulus	# E : Latency 2, extra map slot
+	nop				# E : as part of the cmovne
+	bne	mask,2b			# U : U L U L
+#else
+	srl	divisor,1,divisor	# U :
+	cmovne	compare,tmp1,modulus	# E : Latency 2, extra map slot
+	nop				# E : as part of the cmovne
+	bne	mask,2b			# U : U L L U
+#endif
+
+9:	ldq	$1, 0($30)		# L :
+	ldq	$2, 8($30)		# L :
+	nop				# E :
+	nop				# E : U U L L
+
+	ldq	$0,16($30)		# L :
+	ldq	tmp1,24($30)		# L :
+	nop				# E :
+	nop				# E :
+
+#ifdef DIV
+	DIV_ONLY(ldq tmp2,32($30))	# L :
+#else
+	nop				# E :
+#endif
+	addq	$30,STACK,$30		# E :
+	ret	$31,($23),1		# L0 : L U U L
+	.end	ufunction
+EXPORT_SYMBOL(ufunction)
+
+/*
+ * Uhh.. Ugly signed division. I'd rather not have it at all, but
+ * it's needed in some circumstances. There are different ways to
+ * handle this, really. This does:
+ * 	-a / b = a / -b = -(a / b)
+ *	-a % b = -(a % b)
+ *	a % -b = a % b
+ * which is probably not the best solution, but at least should
+ * have the property that (x/y)*y + (x%y) = x.
+ */
+.align 4
+.globl	sfunction
+.ent	sfunction
+sfunction:
+	subq	$30,STACK,$30		# E :
+	.frame	$30,STACK,$23
+	.prologue 0
+	bis	$24,$25,$28		# E :
+	SLONGIFY($28)			# E :
+	bge	$28,7b			# U :
+
+	stq	$24,0($30)		# L :
+	subq	$31,$24,$28		# E :
+	stq	$25,8($30)		# L :
+	nop				# E : U L U L
+
+	cmovlt	$24,$28,$24	/* abs($24) */ # E : Latency 2, extra map slot
+	nop				# E : as part of the cmov
+	stq	$23,16($30)		# L :
+	subq	$31,$25,$28		# E : U L U L
+
+	stq	tmp1,24($30)		# L :
+	cmovlt	$25,$28,$25	/* abs($25) */ # E : Latency 2, extra map slot
+	nop				# E :
+	bsr	$23,ufunction		# L0: L U L U
+
+	ldq	$24,0($30)		# L :
+	ldq	$25,8($30)		# L :
+	GETSIGN($28)			# E :
+	subq	$31,$27,tmp1		# E : U U L L
+
+	SLONGIFY($28)			# E :
+	ldq	$23,16($30)		# L :
+	cmovlt	$28,tmp1,$27		# E : Latency 2, extra map slot
+	nop				# E : U L L U : as part of the cmov
+
+	ldq	tmp1,24($30)		# L :
+	nop				# E : as part of the cmov
+	addq	$30,STACK,$30		# E :
+	ret	$31,($23),1		# L0 : L U U L
+	.end	sfunction
+EXPORT_SYMBOL(sfunction)