[llvm-extract] Add -keep-const-init commandline option

Summary:
This adds -keep-const-init option to llvm-extract which preserves initializers of
used global constants.

For example:

```
$ cat a.ll
@g = constant i32 0
define i32 @f() {
  %v = load i32, i32* @g
  ret i32 %v
}

$ llvm-extract --func=f a.ll -S -o -
@g = external constant i32
define i32 @f() { .. }

$ llvm-extract --func=f a.ll -keep-const-init -S -o -
@g = constant i32 0
define i32 @f() { .. }
```

This option is useful in checking whether a function that uses a constant global is optimized correctly.

Reviewers: jsji, MaskRay, david2050

Reviewed By: MaskRay

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73833

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/docs/CommandGuide/llvm-extract.rst b/llvm/docs/CommandGuide/llvm-extract.rst
index cd8828b..8e23e70c 100644
--- a/llvm/docs/CommandGuide/llvm-extract.rst
+++ b/llvm/docs/CommandGuide/llvm-extract.rst
@@ -55,6 +55,10 @@ OPTIONS
  bitcode.  All global variables matching the regular expression will be
  extracted.  May be specified multiple times.
 
+**--keep-const-init**
+
+ Preserve the values of constant globals.
+
 **-help**
 
  Print a summary of command line options.