[sledge] Add globalopt pass to remove globals
Summary:
This adds a globalopt optimization pass to sledge.
Consider code like:
```
const char *a_string = "I'm a string";
int an_int = 0;
int c() {
return an_int;
}
int main() {
char *c1 = a_string;
return c();
}
```
When compiled there are 2 levels of indirection. For example
`return an_int` Get's compiled as
```
%0 = load i32, i32* an_int1
ret i32 %0
```
Global opt reduces this (if `an_int` is internal) to just
` ret i32 0`.
Similarly and more importantly
`c1 = a_string;` get's compiled into
```
@.str = private unnamed_addr constant [13 x i8] c"I'm a string\00"
a_string = dso_local global i8* getelementptr inbounds ([13 x i8], [13 x i8]* @.str, i32 0, i32 0)
%c1 = alloca i8*, align 8
%0 = load i8*, i8** a_string, align 8, !dbg !25
store i8* %0, i8** %c1, align 8, !dbg !24
```
So there is a level of indirection between `c1` and `.str` where the string is stored.
With global opt, this gets reduced to:
```
@.str = private unnamed_addr constant [13 x i8] c"I'm a string\00"
%c1 = alloca i8*, align 8
store i8* getelementptr inbounds ([13 x i8], [13 x i8]* @.str, i64 0, i64 0), i8** %c1, align 8, !dbg !23
```
and `a_string` variable gets deleted.
On sledge this has the effect of reducing the complexity of the symbolic heap significantly.
Without this optimisation, running
`sledge.dbg llvm analyze -trace Domain.call global_vars.bc`
Gives prints the following segments:
```
∧ %.str -[)-> ⟨13,{}⟩
* %a_string -[)-> ⟨8,%.str⟩
* %an_int -[)-> ⟨4,0⟩
* %c1 -[)-> ⟨8,%.str⟩
* %retval -[)-> ⟨4,0⟩
```
So there are `an_int` and `a_string` segments, which are redundant.
with the optimisation, the heap looks like:
`∧ %.str -[)-> ⟨13,{}⟩ * %c1 -[)-> ⟨8,%.str⟩ * %retval -[)-> ⟨4,0⟩`,
Where we only have the `.str` segment and the `c1` segment, which are the two we need.
Reviewed By: ngorogiannis
Differential Revision: D15649195
fbshipit-source-id: 5f71e56e8
6 years ago
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/*
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* Copyright (c) 2019-present, Facebook, Inc.
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*
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* This source code is licensed under the MIT license found in the
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* LICENSE file in the root directory of this source tree.
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*/
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/* The symbolic heap for this example should look like:
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^ %.str --> <13,{}> * %c1 --> <8,%.str> * %retval --> <4,0>
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instead of
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^ %.str --> <13,{}>
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* %a_string --> <8,%.str>
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* %an_int --> <4,0>
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* %c1 --> <8,%.str>
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* %retval --> <4,0>
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Which has an_int and a_string indirections. There can be obtained by
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sledge.dbg llvm analyze -trace Domain.call global_vars.bc */
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const char* a_string = "I'm a string";
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int an_int = 0;
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int c() { return an_int; }
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int main() {
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const char* c1 = a_string;
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[sledge] Add globalopt pass to remove globals
Summary:
This adds a globalopt optimization pass to sledge.
Consider code like:
```
const char *a_string = "I'm a string";
int an_int = 0;
int c() {
return an_int;
}
int main() {
char *c1 = a_string;
return c();
}
```
When compiled there are 2 levels of indirection. For example
`return an_int` Get's compiled as
```
%0 = load i32, i32* an_int1
ret i32 %0
```
Global opt reduces this (if `an_int` is internal) to just
` ret i32 0`.
Similarly and more importantly
`c1 = a_string;` get's compiled into
```
@.str = private unnamed_addr constant [13 x i8] c"I'm a string\00"
a_string = dso_local global i8* getelementptr inbounds ([13 x i8], [13 x i8]* @.str, i32 0, i32 0)
%c1 = alloca i8*, align 8
%0 = load i8*, i8** a_string, align 8, !dbg !25
store i8* %0, i8** %c1, align 8, !dbg !24
```
So there is a level of indirection between `c1` and `.str` where the string is stored.
With global opt, this gets reduced to:
```
@.str = private unnamed_addr constant [13 x i8] c"I'm a string\00"
%c1 = alloca i8*, align 8
store i8* getelementptr inbounds ([13 x i8], [13 x i8]* @.str, i64 0, i64 0), i8** %c1, align 8, !dbg !23
```
and `a_string` variable gets deleted.
On sledge this has the effect of reducing the complexity of the symbolic heap significantly.
Without this optimisation, running
`sledge.dbg llvm analyze -trace Domain.call global_vars.bc`
Gives prints the following segments:
```
∧ %.str -[)-> ⟨13,{}⟩
* %a_string -[)-> ⟨8,%.str⟩
* %an_int -[)-> ⟨4,0⟩
* %c1 -[)-> ⟨8,%.str⟩
* %retval -[)-> ⟨4,0⟩
```
So there are `an_int` and `a_string` segments, which are redundant.
with the optimisation, the heap looks like:
`∧ %.str -[)-> ⟨13,{}⟩ * %c1 -[)-> ⟨8,%.str⟩ * %retval -[)-> ⟨4,0⟩`,
Where we only have the `.str` segment and the `c1` segment, which are the two we need.
Reviewed By: ngorogiannis
Differential Revision: D15649195
fbshipit-source-id: 5f71e56e8
6 years ago
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return c();
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}
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