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A Brief Introduction to Using LLVM Nick Sumner Spring 2013 What is LLVM? A compiler? What is LLVM? A compiler? A set of formats, libraries and tools. What is LLVM? A compiler? A set of formats, libraries and tools. A

  1. A Brief Introduction to Using LLVM Nick Sumner Spring 2013

  2. What is LLVM? ● A compiler?

  3. What is LLVM? ● A compiler? ● A set of formats, libraries and tools.

  4. What is LLVM? ● A compiler? ● A set of formats, libraries and tools. – A simple, typed IR ( bitcode ) – Program analysis / optimization libraries – Machine code generation libraries – Tools that compose the libraries to perform task

  5. What is LLVM? ● A compiler? ● A set of formats, libraries and tools. – A simple, typed IR ( bitcode ) – Program analysis / optimization libraries – Machine code generation libraries – Tools that compose the libraries to perform tasks ● Easy to add / remove / change functionality

  6. How will you be using it? ● Compiling programs to bitcode: clang -g -c -emit-llvm <sourcefile> -o <bitcode>.bc

  7. How will you be using it? ● Compiling programs to bitcode: clang -g -c -emit-llvm <sourcefile> -o <bitcode>.bc ● Analyzing the bitcode: opt -load <plugin>.so --<plugin> -analyze <bitcode>.bc

  8. How will you be using it? ● Compiling programs to bitcode: clang -g -c -emit-llvm <sourcefile> -o <bitcode>.bc ● Analyzing the bitcode: opt -load <plugin>.so --<plugin> -analyze <bitcode>.bc ● Reporting properties of the program: [main] : [A], [C], [F] [A] : [B] [C] : [E], [D]

  9. What is LLVM Bitcode? ● A (Relatively) Simple IR @str = private constant [6 x i8] c"Hello\00" #include<stdio.h> define void @foo(i32 %e) { %1 = icmp eq i32 %e, 0 void br i1 %1, label %._crit_edge, label %.lr.ph foo(unsigned e) { for (unsigned i = 0; i < e; ++i) { .lr.ph: ; preds = %.lr.ph, %0 printf("Hello\n"); %i = phi i32 [ %2, %.lr.ph ], [ 0, %0 ] Code } %str1 = getelementptr } [6 x i8]* @str, i64 0, i64 0 %puts = tail call i32 @puts(i8* %str1) IR int %2 = add i32 %i, 1 main(int argc, char **argv) { %cond = icmp eq i32 %2, %e foo(argc); br i1 %cond, label %.exit, label %.lr.ph return 0; } .exit: ; preds = %.lr.ph, %0 ret void } clang -c -emit-llvm define i32 @main(i32 %argc, i8** %argv) { tail call void @foo(i32 %argc) (and llvm-dis) ret i32 0 }

  10. What is LLVM Bitcode? ● A (Relatively) Simple IR @str = private constant [6 x i8] c"Hello\00" #include<stdio.h> define void @foo(i32 %e) { %1 = icmp eq i32 %e, 0 void br i1 %1, label %._crit_edge, label %.lr.ph foo(unsigned e) { for (unsigned i = 0; i < e; ++i) { .lr.ph: ; preds = %.lr.ph, %0 printf("Hello\n"); %i = phi i32 [ %2, %.lr.ph ], [ 0, %0 ] } %str1 = getelementptr } [6 x i8]* @str, i64 0, i64 0 %puts = tail call i32 @puts(i8* %str1) int %2 = add i32 %i, 1 main(int argc, char **argv) { %cond = icmp eq i32 %2, %e foo(argc); br i1 %cond, label %.exit, label %.lr.ph return 0; } .exit: ; preds = %.lr.ph, %0 ret void } clang -c -emit-llvm define i32 @main(i32 %argc, i8** %argv) { tail call void @foo(i32 %argc) (and llvm-dis) ret i32 0 }

  11. What is LLVM Bitcode? ● A (Relatively) Simple IR @str = private constant [6 x i8] c"Hello\00" #include<stdio.h> define void @foo(i32 %e) { %1 = icmp eq i32 %e, 0 void br i1 %1, label %._crit_edge, label %.lr.ph foo(unsigned e) { for (unsigned i = 0; i < e; ++i) { .lr.ph: ; preds = %.lr.ph, %0 printf("Hello\n"); %i = phi i32 [ %2, %.lr.ph ], [ 0, %0 ] } %str1 = getelementptr } [6 x i8]* @str, i64 0, i64 0 %puts = tail call i32 @puts(i8* %str1) int %2 = add i32 %i, 1 main(int argc, char **argv) { %cond = icmp eq i32 %2, %e foo(argc); br i1 %cond, label %.exit, label %.lr.ph return 0; } .exit: ; preds = %.lr.ph, %0 ret void } define i32 @main(i32 %argc, i8** %argv) { Functions tail call void @foo(i32 %argc) ret i32 0 }

  12. What is LLVM Bitcode? ● A (Relatively) Simple IR @str = private constant [6 x i8] c"Hello\00" #include<stdio.h> define void @foo(i32 %e) { %1 = icmp eq i32 %e, 0 void br i1 %1, label %._crit_edge, label %.lr.ph foo(unsigned e) { for (unsigned i = 0; i < e; ++i) { .lr.ph: ; preds = %.lr.ph, %0 printf("Hello\n"); %i = phi i32 [ %2, %.lr.ph ], [ 0, %0 ] } %str1 = getelementptr } [6 x i8]* @str, i64 0, i64 0 %puts = tail call i32 @puts(i8* %str1) int %2 = add i32 %i, 1 main(int argc, char **argv) { %cond = icmp eq i32 %2, %e foo(argc); br i1 %cond, label %.exit, label %.lr.ph return 0; } .exit: ; preds = %.lr.ph, %0 ret void } define i32 @main(i32 %argc, i8** %argv) { Basic Blocks tail call void @foo(i32 %argc) ret i32 0 }

  13. What is LLVM Bitcode? ● A (Relatively) Simple IR @str = private constant [6 x i8] c"Hello\00" #include<stdio.h> define void @foo(i32 %e) { %1 = icmp eq i32 %e, 0 void br i1 %1, label %._crit_edge, label %.lr.ph foo(unsigned e) { for (unsigned i = 0; i < e; ++i) { .lr.ph: ; preds = %.lr.ph, %0 printf("Hello\n"); %i = phi i32 [ %2, %.lr.ph ], [ 0, %0 ] } %str1 = getelementptr } [6 x i8]* @str, i64 0, i64 0 labels & predecessors %puts = tail call i32 @puts(i8* %str1) int %2 = add i32 %i, 1 main(int argc, char **argv) { %cond = icmp eq i32 %2, %e foo(argc); br i1 %cond, label %.exit, label %.lr.ph return 0; } .exit: ; preds = %.lr.ph, %0 ret void } define i32 @main(i32 %argc, i8** %argv) { Basic Blocks tail call void @foo(i32 %argc) ret i32 0 }

  14. What is LLVM Bitcode? ● A (Relatively) Simple IR @str = private constant [6 x i8] c"Hello\00" #include<stdio.h> define void @foo(i32 %e) { %1 = icmp eq i32 %e, 0 void br i1 %1, label %._crit_edge, label %.lr.ph foo(unsigned e) { for (unsigned i = 0; i < e; ++i) { .lr.ph: ; preds = %.lr.ph, %0 printf("Hello\n"); %i = phi i32 [ %2, %.lr.ph ], [ 0, %0 ] } %str1 = getelementptr } [6 x i8]* @str, i64 0, i64 0 branches & successors %puts = tail call i32 @puts(i8* %str1) int %2 = add i32 %i, 1 main(int argc, char **argv) { %cond = icmp eq i32 %2, %e foo(argc); br i1 %cond, label %.exit, label %.lr.ph return 0; } .exit: ; preds = %.lr.ph, %0 ret void } define i32 @main(i32 %argc, i8** %argv) { Basic Blocks tail call void @foo(i32 %argc) ret i32 0 }

  15. What is LLVM Bitcode? ● A (Relatively) Simple IR @str = private constant [6 x i8] c"Hello\00" #include<stdio.h> define void @foo(i32 %e) { %1 = icmp eq i32 %e, 0 void br i1 %1, label %._crit_edge, label %.lr.ph foo(unsigned e) { for (unsigned i = 0; i < e; ++i) { .lr.ph: ; preds = %.lr.ph, %0 printf("Hello\n"); %i = phi i32 [ %2, %.lr.ph ], [ 0, %0 ] } %str1 = getelementptr } [6 x i8]* @str, i64 0, i64 0 %puts = tail call i32 @puts(i8* %str1) int %2 = add i32 %i, 1 main(int argc, char **argv) { %cond = icmp eq i32 %2, %e foo(argc); br i1 %cond, label %.exit, label %.lr.ph return 0; } .exit: ; preds = %.lr.ph, %0 ret void } define i32 @main(i32 %argc, i8** %argv) { Instructions tail call void @foo(i32 %argc) ret i32 0 }

  16. Inspecting Bitcode ● LLVM libraries help examine the bitcode – Easy to examine and/or manipulate

  17. Inspecting Bitcode ● LLVM libraries help examine the bitcode – Easy to examine and/or manipulate Module &module = ...; for (Function &fun : module) { for (BasicBlock &bb : fun) { for (Instruction &i : bb) { Iterate over the: ● Functions in a Module ● BasicBlocks in a Function ● Instructions in a BasicBlock ...

  18. Inspecting Bitcode ● LLVM libraries help examine the bitcode – Easy to examine and/or manipulate – Many helpers (e.g. CallSite, outs(), dyn_cast) Module &module = ...; for (Function &fun : module) { for (BasicBlock &bb : fun) { for (Instruction &i : bb) { CallSite helps you extract CallSite cs(&i); information from Call and if (!cs.getInstruction()) { continue; Invoke instructions. } ...

  19. Inspecting Bitcode ● LLVM libraries help examine the bitcode – Easy to examine and/or manipulate – Many helpers (e.g. CallSite, outs(), dyn_cast) Module &module = ...; for (Function &fun : module) { for (BasicBlock &bb : fun) { for (Instruction &i : bb) { CallSite cs(&i); if (!cs.getInstruction()) { continue; } outs() << "Found a function call: " << i << "\n"; ...

  20. Inspecting Bitcode ● LLVM libraries help examine the bitcode – Easy to examine and/or manipulate – Many helpers (e.g. CallSite, outs(), dyn_cast) Module &module = ...; for (Function &fun : module) { for (BasicBlock &bb : fun) { dyn_cast() efficiently checks for (Instruction &i : bb) { the runtime types of LLVM CallSite cs(&i); IR components. if (!cs.getInstruction()) { continue; } outs() << "Found a function call: " << i << "\n"; Value *called = cs.getCalledValue()->stripPointerCasts(); if (Function *f = dyn_cast<Function>(called)) { outs() << "Direct call to function: " << f->getName() << "\n"; ...

  21. Dealing with SSA ● You may ask where certain values came from – Useful for tracking dependencies – “Where was this variable defined?”

  22. Dealing with SSA ● You may ask where certain values came from ● LLVM IR is in SSA form – How many acronyms can I fit into one line? – What does this mean? – Why does it matter?

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