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, 55 insertions, 0 deletions

diff --git a/arch/powerpc/lib/div64.S b/arch/powerpc/lib/div64.S
new file mode 100644
index 000000000..3d5426e7d
--- /dev/null
+++ b/arch/powerpc/lib/div64.S
@@ -0,0 +1,55 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Divide a 64-bit unsigned number by a 32-bit unsigned number.
+ * This routine assumes that the top 32 bits of the dividend are
+ * non-zero to start with.
+ * On entry, r3 points to the dividend, which get overwritten with
+ * the 64-bit quotient, and r4 contains the divisor.
+ * On exit, r3 contains the remainder.
+ *
+ * Copyright (C) 2002 Paul Mackerras, IBM Corp.
+ */
+#include <asm/ppc_asm.h>
+#include <asm/processor.h>
+
+_GLOBAL(__div64_32)
+	lwz	r5,0(r3)	# get the dividend into r5/r6
+	lwz	r6,4(r3)
+	cmplw	r5,r4
+	li	r7,0
+	li	r8,0
+	blt	1f
+	divwu	r7,r5,r4	# if dividend.hi >= divisor,
+	mullw	r0,r7,r4	# quotient.hi = dividend.hi / divisor
+	subf.	r5,r0,r5	# dividend.hi %= divisor
+	beq	3f
+1:	mr	r11,r5		# here dividend.hi != 0
+	andis.	r0,r5,0xc000
+	bne	2f
+	cntlzw	r0,r5		# we are shifting the dividend right
+	li	r10,-1		# to make it < 2^32, and shifting
+	srw	r10,r10,r0	# the divisor right the same amount,
+	addc	r9,r4,r10	# rounding up (so the estimate cannot
+	andc	r11,r6,r10	# ever be too large, only too small)
+	andc	r9,r9,r10
+	addze	r9,r9
+	or	r11,r5,r11
+	rotlw	r9,r9,r0
+	rotlw	r11,r11,r0
+	divwu	r11,r11,r9	# then we divide the shifted quantities
+2:	mullw	r10,r11,r4	# to get an estimate of the quotient,
+	mulhwu	r9,r11,r4	# multiply the estimate by the divisor,
+	subfc	r6,r10,r6	# take the product from the divisor,
+	add	r8,r8,r11	# and add the estimate to the accumulated
+	subfe.	r5,r9,r5	# quotient
+	bne	1b
+3:	cmplw	r6,r4
+	blt	4f
+	divwu	r0,r6,r4	# perform the remaining 32-bit division
+	mullw	r10,r0,r4	# and get the remainder
+	add	r8,r8,r0
+	subf	r6,r10,r6
+4:	stw	r7,0(r3)	# return the quotient in *r3
+	stw	r8,4(r3)
+	mr	r3,r6		# return the remainder in r3
+	blr