Date:   Tue, 1 Feb 2022 21:12:54 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     German Gomez <german.gomez@....com>
Cc:     linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH] perf symbols: Remove demangle-rust files as superfluous

Em Tue, Feb 01, 2022 at 06:50:53PM +0000, German Gomez escreveu:
> Demangling of Rust legacy symbols has been working through libbfd for
> some time now, making these files no longer needed, so remove them.

Can this be made dependent of the versions of the components that
provide this feature?

- Arnaldo
 
> Signed-off-by: German Gomez <german.gomez@....com>
> ---
> Before and after the commit:
> 
> $ perf record rustc
> $ perf report
> 
> 2.98%  rustc    rustc                                [.] <std::collections::hash::map::DefaultHasher as core::hash::Hasher>::write
> 2.77%  rustc    rustc                                [.] std::collections::hash::map::HashMap<K,V,S>::entry
> 2.09%  rustc    rustc                                [.] <<toml::value::Value as serde::de::Deserialize>::deserialize::ValueVisitor as serde::de::Visitor>::visit_map
> 2.05%  rustc    rustc                                [.] <toml::de::MapVisitor as serde::de::MapAccess>::next_key_seed
> 1.52%  rustc    rustc                                [.] std::collections::hash::map::HashMap<K,V,S>::get
> 0.59%  rustc    rustc                                [.] <toml::de::MapVisitor as serde::de::MapAccess>::next_value_seed
> ---
>  tools/perf/util/Build           |   1 -
>  tools/perf/util/demangle-rust.c | 269 --------------------------------
>  tools/perf/util/demangle-rust.h |   8 -
>  tools/perf/util/symbol-elf.c    |   7 -
>  4 files changed, 285 deletions(-)
>  delete mode 100644 tools/perf/util/demangle-rust.c
>  delete mode 100644 tools/perf/util/demangle-rust.h
> 
> diff --git a/tools/perf/util/Build b/tools/perf/util/Build
> index 2a403cefc..e59788ef6 100644
> --- a/tools/perf/util/Build
> +++ b/tools/perf/util/Build
> @@ -193,7 +193,6 @@ perf-$(CONFIG_LIBCAP) += cap.o
>  
>  perf-y += demangle-ocaml.o
>  perf-y += demangle-java.o
> -perf-y += demangle-rust.o
>  
>  ifdef CONFIG_JITDUMP
>  perf-$(CONFIG_LIBELF) += jitdump.o
> diff --git a/tools/perf/util/demangle-rust.c b/tools/perf/util/demangle-rust.c
> deleted file mode 100644
> index a659fc69f..000000000
> --- a/tools/perf/util/demangle-rust.c
> +++ /dev/null
> @@ -1,269 +0,0 @@
> -// SPDX-License-Identifier: GPL-2.0
> -#include <string.h>
> -#include "debug.h"
> -
> -#include "demangle-rust.h"
> -
> -/*
> - * Mangled Rust symbols look like this:
> - *
> - *     _$LT$std..sys..fd..FileDesc$u20$as$u20$core..ops..Drop$GT$::drop::hc68340e1baa4987a
> - *
> - * The original symbol is:
> - *
> - *     <std::sys::fd::FileDesc as core::ops::Drop>::drop
> - *
> - * The last component of the path is a 64-bit hash in lowercase hex, prefixed
> - * with "h". Rust does not have a global namespace between crates, an illusion
> - * which Rust maintains by using the hash to distinguish things that would
> - * otherwise have the same symbol.
> - *
> - * Any path component not starting with a XID_Start character is prefixed with
> - * "_".
> - *
> - * The following escape sequences are used:
> - *
> - *     ","  =>  $C$
> - *     "@"  =>  $SP$
> - *     "*"  =>  $BP$
> - *     "&"  =>  $RF$
> - *     "<"  =>  $LT$
> - *     ">"  =>  $GT$
> - *     "("  =>  $LP$
> - *     ")"  =>  $RP$
> - *     " "  =>  $u20$
> - *     "'"  =>  $u27$
> - *     "["  =>  $u5b$
> - *     "]"  =>  $u5d$
> - *     "~"  =>  $u7e$
> - *
> - * A double ".." means "::" and a single "." means "-".
> - *
> - * The only characters allowed in the mangled symbol are a-zA-Z0-9 and _.:$
> - */
> -
> -static const char *hash_prefix = "::h";
> -static const size_t hash_prefix_len = 3;
> -static const size_t hash_len = 16;
> -
> -static bool is_prefixed_hash(const char *start);
> -static bool looks_like_rust(const char *sym, size_t len);
> -static bool unescape(const char **in, char **out, const char *seq, char value);
> -
> -/*
> - * INPUT:
> - *     sym: symbol that has been through BFD-demangling
> - *
> - * This function looks for the following indicators:
> - *
> - *  1. The hash must consist of "h" followed by 16 lowercase hex digits.
> - *
> - *  2. As a sanity check, the hash must use between 5 and 15 of the 16 possible
> - *     hex digits. This is true of 99.9998% of hashes so once in your life you
> - *     may see a false negative. The point is to notice path components that
> - *     could be Rust hashes but are probably not, like "haaaaaaaaaaaaaaaa". In
> - *     this case a false positive (non-Rust symbol has an important path
> - *     component removed because it looks like a Rust hash) is worse than a
> - *     false negative (the rare Rust symbol is not demangled) so this sets the
> - *     balance in favor of false negatives.
> - *
> - *  3. There must be no characters other than a-zA-Z0-9 and _.:$
> - *
> - *  4. There must be no unrecognized $-sign sequences.
> - *
> - *  5. There must be no sequence of three or more dots in a row ("...").
> - */
> -bool
> -rust_is_mangled(const char *sym)
> -{
> -	size_t len, len_without_hash;
> -
> -	if (!sym)
> -		return false;
> -
> -	len = strlen(sym);
> -	if (len <= hash_prefix_len + hash_len)
> -		/* Not long enough to contain "::h" + hash + something else */
> -		return false;
> -
> -	len_without_hash = len - (hash_prefix_len + hash_len);
> -	if (!is_prefixed_hash(sym + len_without_hash))
> -		return false;
> -
> -	return looks_like_rust(sym, len_without_hash);
> -}
> -
> -/*
> - * A hash is the prefix "::h" followed by 16 lowercase hex digits. The hex
> - * digits must comprise between 5 and 15 (inclusive) distinct digits.
> - */
> -static bool is_prefixed_hash(const char *str)
> -{
> -	const char *end;
> -	bool seen[16];
> -	size_t i;
> -	int count;
> -
> -	if (strncmp(str, hash_prefix, hash_prefix_len))
> -		return false;
> -	str += hash_prefix_len;
> -
> -	memset(seen, false, sizeof(seen));
> -	for (end = str + hash_len; str < end; str++)
> -		if (*str >= '0' && *str <= '9')
> -			seen[*str - '0'] = true;
> -		else if (*str >= 'a' && *str <= 'f')
> -			seen[*str - 'a' + 10] = true;
> -		else
> -			return false;
> -
> -	/* Count how many distinct digits seen */
> -	count = 0;
> -	for (i = 0; i < 16; i++)
> -		if (seen[i])
> -			count++;
> -
> -	return count >= 5 && count <= 15;
> -}
> -
> -static bool looks_like_rust(const char *str, size_t len)
> -{
> -	const char *end = str + len;
> -
> -	while (str < end)
> -		switch (*str) {
> -		case '$':
> -			if (!strncmp(str, "$C$", 3))
> -				str += 3;
> -			else if (!strncmp(str, "$SP$", 4)
> -					|| !strncmp(str, "$BP$", 4)
> -					|| !strncmp(str, "$RF$", 4)
> -					|| !strncmp(str, "$LT$", 4)
> -					|| !strncmp(str, "$GT$", 4)
> -					|| !strncmp(str, "$LP$", 4)
> -					|| !strncmp(str, "$RP$", 4))
> -				str += 4;
> -			else if (!strncmp(str, "$u20$", 5)
> -					|| !strncmp(str, "$u27$", 5)
> -					|| !strncmp(str, "$u5b$", 5)
> -					|| !strncmp(str, "$u5d$", 5)
> -					|| !strncmp(str, "$u7e$", 5))
> -				str += 5;
> -			else
> -				return false;
> -			break;
> -		case '.':
> -			/* Do not allow three or more consecutive dots */
> -			if (!strncmp(str, "...", 3))
> -				return false;
> -			/* Fall through */
> -		case 'a' ... 'z':
> -		case 'A' ... 'Z':
> -		case '0' ... '9':
> -		case '_':
> -		case ':':
> -			str++;
> -			break;
> -		default:
> -			return false;
> -		}
> -
> -	return true;
> -}
> -
> -/*
> - * INPUT:
> - *     sym: symbol for which rust_is_mangled(sym) returns true
> - *
> - * The input is demangled in-place because the mangled name is always longer
> - * than the demangled one.
> - */
> -void
> -rust_demangle_sym(char *sym)
> -{
> -	const char *in;
> -	char *out;
> -	const char *end;
> -
> -	if (!sym)
> -		return;
> -
> -	in = sym;
> -	out = sym;
> -	end = sym + strlen(sym) - (hash_prefix_len + hash_len);
> -
> -	while (in < end)
> -		switch (*in) {
> -		case '$':
> -			if (!(unescape(&in, &out, "$C$", ',')
> -					|| unescape(&in, &out, "$SP$", '@')
> -					|| unescape(&in, &out, "$BP$", '*')
> -					|| unescape(&in, &out, "$RF$", '&')
> -					|| unescape(&in, &out, "$LT$", '<')
> -					|| unescape(&in, &out, "$GT$", '>')
> -					|| unescape(&in, &out, "$LP$", '(')
> -					|| unescape(&in, &out, "$RP$", ')')
> -					|| unescape(&in, &out, "$u20$", ' ')
> -					|| unescape(&in, &out, "$u27$", '\'')
> -					|| unescape(&in, &out, "$u5b$", '[')
> -					|| unescape(&in, &out, "$u5d$", ']')
> -					|| unescape(&in, &out, "$u7e$", '~'))) {
> -				pr_err("demangle-rust: unexpected escape sequence");
> -				goto done;
> -			}
> -			break;
> -		case '_':
> -			/*
> -			 * If this is the start of a path component and the next
> -			 * character is an escape sequence, ignore the
> -			 * underscore. The mangler inserts an underscore to make
> -			 * sure the path component begins with a XID_Start
> -			 * character.
> -			 */
> -			if ((in == sym || in[-1] == ':') && in[1] == '$')
> -				in++;
> -			else
> -				*out++ = *in++;
> -			break;
> -		case '.':
> -			if (in[1] == '.') {
> -				/* ".." becomes "::" */
> -				*out++ = ':';
> -				*out++ = ':';
> -				in += 2;
> -			} else {
> -				/* "." becomes "-" */
> -				*out++ = '-';
> -				in++;
> -			}
> -			break;
> -		case 'a' ... 'z':
> -		case 'A' ... 'Z':
> -		case '0' ... '9':
> -		case ':':
> -			*out++ = *in++;
> -			break;
> -		default:
> -			pr_err("demangle-rust: unexpected character '%c' in symbol\n",
> -				*in);
> -			goto done;
> -		}
> -
> -done:
> -	*out = '\0';
> -}
> -
> -static bool unescape(const char **in, char **out, const char *seq, char value)
> -{
> -	size_t len = strlen(seq);
> -
> -	if (strncmp(*in, seq, len))
> -		return false;
> -
> -	**out = value;
> -
> -	*in += len;
> -	*out += 1;
> -
> -	return true;
> -}
> diff --git a/tools/perf/util/demangle-rust.h b/tools/perf/util/demangle-rust.h
> deleted file mode 100644
> index 2fca618b1..000000000
> --- a/tools/perf/util/demangle-rust.h
> +++ /dev/null
> @@ -1,8 +0,0 @@
> -/* SPDX-License-Identifier: GPL-2.0 */
> -#ifndef __PERF_DEMANGLE_RUST
> -#define __PERF_DEMANGLE_RUST 1
> -
> -bool rust_is_mangled(const char *str);
> -void rust_demangle_sym(char *str);
> -
> -#endif /* __PERF_DEMANGLE_RUST */
> diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
> index 31cd59a2b..3ed0bef81 100644
> --- a/tools/perf/util/symbol-elf.c
> +++ b/tools/perf/util/symbol-elf.c
> @@ -14,7 +14,6 @@
>  #include "symsrc.h"
>  #include "demangle-ocaml.h"
>  #include "demangle-java.h"
> -#include "demangle-rust.h"
>  #include "machine.h"
>  #include "vdso.h"
>  #include "debug.h"
> @@ -258,12 +257,6 @@ static char *demangle_sym(struct dso *dso, int kmodule, const char *elf_name)
>  			demangled = java_demangle_sym(elf_name, JAVA_DEMANGLE_NORET);
>  		}
>  	}
> -	else if (rust_is_mangled(demangled))
> -		/*
> -		    * Input to Rust demangling is the BFD-demangled
> -		    * name which it Rust-demangles in place.
> -		    */
> -		rust_demangle_sym(demangled);
>  
>  	return demangled;
>  }
> -- 
> 2.25.1

-- 

- Arnaldo

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