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, 99 insertions, 0 deletions

diff --git a/tools/perf/util/branch.h b/tools/perf/util/branch.h
new file mode 100644
index 000000000..a1d473649
--- /dev/null
+++ b/tools/perf/util/branch.h
@@ -0,0 +1,99 @@
+#ifndef _PERF_BRANCH_H
+#define _PERF_BRANCH_H 1
+/*
+ * The linux/stddef.h isn't need here, but is needed for __always_inline used
+ * in files included from uapi/linux/perf_event.h such as
+ * /usr/include/linux/swab.h and /usr/include/linux/byteorder/little_endian.h,
+ * detected in at least musl libc, used in Alpine Linux. -acme
+ */
+#include <stdio.h>
+#include <linux/perf_event.h>
+#include <linux/types.h>
+#include "util/map_symbol.h"
+#include "util/sample.h"
+
+struct branch_flags {
+	union {
+		u64 value;
+		struct {
+			u64 mispred:1;
+			u64 predicted:1;
+			u64 in_tx:1;
+			u64 abort:1;
+			u64 cycles:16;
+			u64 type:4;
+			u64 spec:2;
+			u64 new_type:4;
+			u64 priv:3;
+			u64 not_taken:1;
+			u64 reserved:30;
+		};
+	};
+};
+
+struct branch_info {
+	struct addr_map_symbol from;
+	struct addr_map_symbol to;
+	struct branch_flags    flags;
+	u64		       branch_stack_cntr;
+	char		       *srcline_from;
+	char		       *srcline_to;
+};
+
+struct branch_entry {
+	u64			from;
+	u64			to;
+	struct branch_flags	flags;
+};
+
+struct branch_stack {
+	u64			nr;
+	u64			hw_idx;
+	struct branch_entry	entries[];
+};
+
+/*
+ * The hw_idx is only available when PERF_SAMPLE_BRANCH_HW_INDEX is applied.
+ * Otherwise, the output format of a sample with branch stack is
+ * struct branch_stack {
+ *	u64			nr;
+ *	struct branch_entry	entries[0];
+ * }
+ * Check whether the hw_idx is available,
+ * and return the corresponding pointer of entries[0].
+ */
+static inline struct branch_entry *perf_sample__branch_entries(struct perf_sample *sample)
+{
+	u64 *entry = (u64 *)sample->branch_stack;
+
+	if (entry == NULL)
+		return NULL;
+
+	entry++;
+	if (sample->no_hw_idx)
+		return (struct branch_entry *)entry;
+	return (struct branch_entry *)(++entry);
+}
+
+struct branch_type_stat {
+	bool	branch_to;
+	u64	counts[PERF_BR_MAX];
+	u64	new_counts[PERF_BR_NEW_MAX];
+	u64	cond_fwd;
+	u64	cond_bwd;
+	u64	cross_4k;
+	u64	cross_2m;
+};
+
+void branch_type_count(struct branch_type_stat *st, struct branch_flags *flags,
+		       u64 from, u64 to);
+
+const char *branch_type_name(int type);
+const char *branch_new_type_name(int new_type);
+const char *get_branch_type(struct branch_entry *e);
+void branch_type_stat_display(FILE *fp, const struct branch_type_stat *st);
+int branch_type_str(const struct branch_type_stat *st, char *bf, int bfsize);
+
+const char *branch_spec_desc(int spec);
+
+#endif /* _PERF_BRANCH_H */