How to replace a piece of text only if it doesn't appear elsewhere in the file?

Question

Suppose I have some text like this (output of objdump -d):

   0:   0f a2                   cpuid  
   2:   a9 01 00 00 00          test   eax,0x1
   7:   74 01                   je     a <myFunc+0xa>
   9:   c3                      ret    
   a:   0f 0b                   ud2a

I'd like to replace the text like ^ +[0-9a-f]+: with corresponding number of spaces (so as to preserve the length), but only if the part before : isn't mentioned anywhere else (as a word, i.e. enclosed in word boundaries). E.g. in the above example the labels 0,2,7,9 would be replaced with a space and a would remain intact (since it's mentioned in the third line).

Here's how the above example would look after processing:

        0f a2                   cpuid  
        a9 01 00 00 00          test   eax,0x1
        74 01                   je     a <myFunc+0xa>
        c3                      ret    
   a:   0f 0b                   ud2a

Is there any nicer way to do it in shell/vim than counting occurrences of the labels and then processing the lines based on these counts?

My current code processes 2300-line file in 3 minutes (on Intel Atom CPU), which is much too long:

#!/bin/bash -e

if [ $# -ne 2 ]; then
    echo "Usage: $0 infile outfile" >&2
    exit 1
fi

file="$1"
outfile="$2"

cp "$file" "$outfile"

labelLength=$(sed -n '/^ \+\([0-9a-f]\+\):.*/{s@@\1@p;q}' "$file"|wc -c)
replacement=$(printf %${labelLength}c ' ')

sed 's@^ \+\([0-9a-f]\+\):.*@\1@' "$file" | while read label
do
    if [ $(grep -c "\<$label\>" "$file") = 1 ]; then
        sed -i "s@\<$label\>:@$replacement@" "$outfile"
    fi
done

Answer 1

A Perl solution:

$ perl -lne '$k{"$_:"}++ for split(/\b/); push @l,$_; }{
   map{s/\S+:/$k{$&}<2 ? " " x length($&) : $&/e; print}@l;' file
     0f a2                   cpuid  
     a9 01 00 00 00          test   eax,0x1
     74 01                   je     a <myFunc+0xa>
     c3                      ret    
a:   0f 0b                   ud2a

This is equivalent to:

#!/usr/bin/perl
use strict;
my %wordsHash;
my @lines;
## Read the input file line by line, saving each
## line as $_. This is what 'perl -n` means.
while (<>) {
    ## Remove trailing newlines. This is done
    ## by -l in the one liner.
    chomp;
    ## Split the current line on word boundaries
    my @wordsArray=split(/\b/);
    ## Save each word + ":" as a key in the hash %wordsHash,
    ## incrementing the value by one each time the word
    ## is seen.
    foreach my $word (@wordsArray) {
        $wordsHash{"$word:"}++;
    }
    ## Add the line to the array @lines
    push @lines, $_;
}

## After the file has been read. ('}{' in the one-liner)
## Iterate over each line in @lines ( map{}@l in the one-liner)
foreach my $line (@lines) {
    ## Grab the first set of non-whitespace
    ## characters until the 1st ':'
    $line=~/\S+:/;
    ## $& is whatever was matched
    my $match=$&;
    ## If the match was seen only once
    ## as a word (all will be seen at least once)
    if ($wordsHash{$match}<2) {
        ## The replacement is as many spaces
        ## as $match has characters.
        my $rep = " " x length($match);
        ## Replace it in the line
        $line=~s/$match/$rep/;
    }
    ## Print the line
    print "$line\n";;
}

Answer 2

Reading between the lines, I assume you want to make your disassembly more readable by removing addresses on lines that are not referred to by jump and similar instructions. This awk assumes the number in the last but one column is an address when the last column begins "<". It reads the disassembly once, remembering all such addresses in an array, then a second time replacing the address at the start of the line if it didn't appear.

$ objdump -d /bin/ls >/tmp/a
$ awk '
NR==FNR { if($NF ~ /</) address[$(NF-1)] = 1; next }
$1 ~ /:/ { if(!address[substr($1,1,length($1)-1)]){
              i = index($0,":")
              printf "%*s%s\n",i," ",substr($0,i+1)
              next }
        }
{ print }
' /tmp/a /tmp/a