I would like to auto-generate Load and InsertElement Instruction through llvm pass.
My problem is how to put Load Instructions such as %4, %6, %8, %10 into InsertElement Instructions, like my goal of LLVM-IR code:
%4 = load i32, i32* %1, align 4
%5 = insertelement <4 x i32> undef, i32 %4, i32 0
%6 = load i32, i32* %1, align 4
%7 = insertelement <4 x i32> %5, i32 %6, i32 1
%8 = load i32, i32* %1, align 4
%9 = insertelement <4 x i32> %7, i32 %8, i32 2
%10 = load i32, i32* %1, align 4
%11 = insertelement <4 x i32> %9, i32 %10, i32 3
and my pass:
if (auto *op = dyn_cast<LoadInst>(&I)) {
...
Value* loadinst_ptr=op->getPointerOperand();
Type* load_ty= loadinst_ptr->getType();
Value* val = UndefValue::get(VectorType::get(load_ty, 4));
for (unsigned i = 0; i < 4; i++)//for 4 copies
{
LoadInst* load_val=builder.CreateLoad(loadinst_ptr);
load_val->setAlignment(4);
//my problem is here, parameter load_val is invalid on CreateInsertElement.
val = builder.CreateInsertElement(val,load_val, builder.getInt32(i));
}
}
here is error message:
opt: /home/shun/llvm-proj/llvm/lib/IR/Instructions.cpp:1748:
llvm::InsertElementInst::InsertElementInst(llvm::Value*, llvm::Value*, llvm::Value*, const llvm::Twine&, llvm::Instruction*):
Assertion `isValidOperands(Vec, Elt, Index) && "Invalid insertelement instruction operands!"' failed.
Related
I want to write an LLVM pass to reduce && in LLVM IR, but I can't find the specific instructions for it in IR. For example,
#include <iostream>
int main(){
bool a = true;
bool b = false;
bool c = a && b;
return 0;
}
and I get the IR,
define dso_local i32 #main() #4 {
%1 = alloca i32, align 4
%2 = alloca i8, align 1
%3 = alloca i8, align 1
%4 = alloca i8, align 1
store i32 0, i32* %1, align 4
store i8 1, i8* %2, align 1
store i8 0, i8* %3, align 1
%5 = load i8, i8* %2, align 1
%6 = trunc i8 %5 to i1
br i1 %6, label %7, label %10
7: ; preds = %0
%8 = load i8, i8* %3, align 1
%9 = trunc i8 %8 to i1
br label %10
10: ; preds = %7, %0
%11 = phi i1 [ false, %0 ], [ %9, %7 ]
%12 = zext i1 %11 to i8
store i8 %12, i8* %4, align 1
ret i32 0
}
but I tried this one,
#include <iostream>
int main(){
int a = 10;
int b = 10;
int c;
c = a && b;
return 0;
}
and I get this
define dso_local i32 #main() #4 {
%1 = alloca i32, align 4
%2 = alloca i32, align 4
%3 = alloca i32, align 4
%4 = alloca i32, align 4
store i32 0, i32* %1, align 4
store i32 10, i32* %2, align 4
store i32 10, i32* %3, align 4
%5 = load i32, i32* %2, align 4
%6 = icmp ne i32 %5, 0
br i1 %6, label %7, label %10
7: ; preds = %0
%8 = load i32, i32* %3, align 4
%9 = icmp ne i32 %8, 0
br label %10
10: ; preds = %7, %0
%11 = phi i1 [ false, %0 ], [ %9, %7 ]
%12 = zext i1 %11 to i32
store i32 %12, i32* %4, align 4
ret i32 0
}
I use LLVM 10 in ubuntu. I'll appreciate any answers or suggestions.
There is no LLVM instruction that specifically corresponds to the && operator. It can and will be translated in different ways depending on the expression and the optimization settings.
When you have optimizations enabled, the operands are side effect free (and not expensive to evaluate) and the whole expression can't be optimized away, clang will usually convert both operands to i1 and apply the logical and operator on them.
When optimizations are disabled or the operands have side effects, it'll usually be translated using branch instructions. That's the case in the two examples you posted.
Note that expr1 && expr2 is semantically equivalent to expr1 ? expr2 : false and you'll generally get the same LLVM code for both.
If you're okay with treating expr1 ? expr2 : false and other equivalent code (for example using if statements) the same as &&, you can try to detect the branching pattern created by them. If you need your pass to also be applicable after optimizations, you'll also have to detect at least the pattern of converting to i1 and anding.
If you only want your transformation to apply to && and nothing else, you simply can't do it at the LLVM level. You'd need an AST transformation at the Clang level.
I am trying to perform -O2 optimisation with LLVM IR obtained by calling CLANG API. Unfortunately, optimisation works only with IR created with manual calls. I have the following function:
int mult_add(int x, int y){
if(x > 2){
return y + 1 + 2;
} else {
return y - 1 + 2;
}
}
And with these calls:
clang -S -emit-llvm main.cpp
opt main.ll -o opt.ll -S -O2
I get the correct result:
define i32 #_Z8mult_addii(i32, i32) local_unnamed_addr #0 {
%3 = icmp sgt i32 %0, 2
%.sink = select i1 %3, i32 3, i32 1
%4 = add nsw i32 %.sink, %1
ret i32 %4
}
Unfortunately, when I do it through LLVM API with legacy::PassManager and legacy::FunctionPassManager optimisation simply does not work and got long ugly code:
define i32 #_Z8mult_addii(i32, i32) #0 {
%3 = alloca i32, align 4
%4 = alloca i32, align 4
%5 = alloca i32, align 4
store i32 %0, i32* %4, align 4
store i32 %1, i32* %5, align 4
%6 = load i32, i32* %4, align 4
%7 = icmp sgt i32 %6, 2
br i1 %7, label %8, label %12
; <label>:8: ; preds = %2
%9 = load i32, i32* %5, align 4
%10 = add nsw i32 %9, 1
%11 = add nsw i32 %10, 2
store i32 %11, i32* %3, align 4
br label %16
; <label>:12: ; preds = %2
%13 = load i32, i32* %5, align 4
%14 = sub nsw i32 %13, 1
%15 = add nsw i32 %14, 2
store i32 %15, i32* %3, align 4
br label %16
; <label>:16: ; preds = %12, %8
%17 = load i32, i32* %3, align 4
ret i32 %17
}
Seems like CLANG creates IR in some unoptimisable state? Because running the passes on a manual created IR works fine.
By the way, PMBuilder.populateModulePassManager is called, here is the code:
legacy::PassManager Passes;
legacy::FunctionPassManager FPasses(M2.get());
AddOptimizationPasses(Passes, FPasses, &(TheJIT->getTargetMachine()), 2, 0);
Passes.add(createPrintModulePass(outs()));
Passes.run(*M2);
And AddOptimizationPasses is stolen and simplified from opt utility:
static void AddOptimizationPasses(legacy::PassManagerBase &MPM,
legacy::FunctionPassManager &FPM,
TargetMachine *TM, unsigned OptLevel,
unsigned SizeLevel) {
FPM.add(createVerifierPass());
PassManagerBuilder Builder;
Builder.OptLevel = OptLevel;
Builder.SizeLevel = SizeLevel;
Builder.Inliner = createFunctionInliningPass(50);
Builder.DisableUnitAtATime = true;//!UnitAtATime;
Builder.DisableUnrollLoops = false;
if (TM)
TM->adjustPassManager(Builder);
//Builder.populateFunctionPassManager(FPM);
Builder.populateModulePassManager(MPM);
}
By the way, initialisation is following:
InitializeAllTargets();
InitializeAllTargetMCs();
InitializeAllAsmPrinters();
Unfortunately, it does not work.
Did you forget to populate the pass manager?
PassManagerBase& PM = ...; // create the pass manager.
PassManagerBuilder PMBuilder;
PMBuilder.OptLevel = 2;
PMBuilder.DisableUnrollLoops = false;
PMBuilder.Inliner = createFunctionInliningPass(50);
PMBuilder.populateModulePassManager(PM);
Module& = ...; // your IR module here
PM.run(M);
Note that a "FunctionPassManager" may not do what you need. You're likely looking for legacy::PassManager instead (which can hold any type of pass).
The DataFlowSanitizer pass on LLVM 3.8.0, 64 bit (Ubuntu 16.04.2) generates the following IR from source:
The source:
test.c
#include <sanitizer/dfsan_interface.h>
int main(void) {
int i = 1;
dfsan_label i_label = dfsan_create_label("i", 0);
dfsan_set_label(i_label, &i, sizeof(i));
return 0;
}
The commands to generate the IR:
clang -c -emit-llvm -fsanitize=dataflow test.c -o test.bc
llvm-dis test.bc
The disassembly:
test.ll
; Function Attrs: nounwind uwtable
define i32 #main() #0 {
entry:
%0 = alloca i16
%retval = alloca i32, align 4
%i = alloca i32, align 4
%1 = alloca i16
%i_label = alloca i16, align 2
store i16 0, i16* %0
store i32 0, i32* %retval, align 4
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
%2 = ptrtoint i32* %i to i64
%3 = and i64 %2, -123145302310913
%4 = mul i64 %3, 2
%5 = inttoptr i64 %4 to i16*
%6 = bitcast i16* %5 to i64*
store i64 0, i64* %6, align 2
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
store i32 1, i32* %i, align 4
%call = call zeroext i16 #dfsan_create_label(i8* getelementptr inbounds ([2 x i8], [2 x i8]* #.str, i32 0, i32 0), i8* null)
store i16 0, i16* %1
store i16 %call, i16* %i_label, align 2
%7 = load i16, i16* %1
%8 = load i16, i16* %i_label, align 2
%9 = bitcast i32* %i to i8*
call void #dfsan_set_label(i16 zeroext %8, i8* %9, i64 4)
ret i32 0
}
I don't understand why the block of instruction I separated out is being generated. Looking at the Transform/Instrumentation/DataFlowsanitizer.cpp, I can't find the code that inserts the instrumentation above. Can anyone explain this behavior?
Let's say I have this function in C/C++:
int foo(int x) {
if (x <= 1) return 1;
return x * foo(x-1);
}
And I compile it with Clang.
Clang generates the following IR:
; Function Attrs: ssp uwtable
define i32 #_Z3fooi(i32 %x) #0 {
%1 = alloca i32, align 4
%2 = alloca i32, align 4
store i32 %x, i32* %2, align 4
%3 = load i32, i32* %2, align 4
%4 = icmp sle i32 %3, 1
br i1 %4, label %5, label %6
; <label>:5 ; preds = %0
store i32 1, i32* %1, align 4
br label %12
; <label>:6 ; preds = %0
%7 = load i32, i32* %2, align 4
%8 = load i32, i32* %2, align 4
%9 = sub nsw i32 %8, 1
%10 = call i32 #_Z3fooi(i32 %9)
%11 = mul nsw i32 %7, %10
store i32 %11, i32* %1, align 4
br label %12
; <label>:12 ; preds = %6, %5
%13 = load i32, i32* %1, align 4
ret i32 %13
}
As you can see, LLVM passes optimizes out the code and creates a "return register" (where I put the return value), and a "return block" (where the return value is effectively returned).
I'm trying to get the same effect, but when I use SROA pass or the Instruction Combining pass, they translate the exits in a phi instruction:
; Function Attrs: nounwind ssp uwtable
define i32 #__HF3fooTi(i32 %x) #0 {
%1 = icmp sle i32 %x, 1
br i1 %1, label %2, label %3
; <label>:2 ; preds = %0
br label %7
; <label>:3 ; preds = %0
%4 = sub nsw i32 %x, 1
%5 = call i32 #__HF3fooTi(i32 %4)
%6 = mul nsw i32 %x, %5
br label %7
; <label>:7 ; preds = %3, %2
%.0 = phi i32 [ 1, %2 ], [ %6, %3 ]
ret i32 %.0
}
My question is: which solution is faster? And which pass is Clang using to achieve this? (In the Clang source files I found the 2 passes I used, and they give me this different result)
I have a c code that calculates the factorial of an int "factorial.c". I compile it to llvm readable code "factorial.ll" and I modify in the compiled llvm code.
The objective is to execute the modified llvm code and to see its output, How can I do this?
It will depend on how your outputted LLVM is assembled and what libraries it links against, but for example executing the following factorial.ll with the shell command lli
$ lli factorial.ll
Factorial of 10 = 3628800
Will execute the main function with the JIT and use the standard printf to output the result to stdout.
#.str = private unnamed_addr constant [22 x i8] c"Factorial of %d = %d\0A\00", align 1
declare i32 #printf(i8*, ...)
define i32 #factorial(i32 %n) nounwind uwtable {
entry:
%n.addr = alloca i32, align 4
store i32 %n, i32* %n.addr, align 4
%0 = load i32* %n.addr, align 4
%cmp = icmp sle i32 %0, 1
br i1 %cmp, label %cond.true, label %cond.false
cond.true: ; preds = %entry
br label %cond.end
cond.false: ; preds = %entry
%1 = load i32* %n.addr, align 4
%2 = load i32* %n.addr, align 4
%sub = sub nsw i32 %2, 1
%call = call i32 #factorial(i32 %sub)
%mul = mul nsw i32 %1, %call
br label %cond.end
cond.end: ; preds = %cond.false, %cond.true
%cond = phi i32 [ 1, %cond.true ], [ %mul, %cond.false ]
ret i32 %cond
}
define i32 #main(i32 %argc, i8** %argv) nounwind uwtable {
entry:
%retval = alloca i32, align 4
%argc.addr = alloca i32, align 4
%argv.addr = alloca i8**, align 8
store i32 0, i32* %retval
store i32 %argc, i32* %argc.addr, align 4
store i8** %argv, i8*** %argv.addr, align 8
%call = call i32 #factorial(i32 10)
%call1 = call i32 (i8*, ...)* #printf(i8* getelementptr inbounds ([22 x i8]* #.str, i32 0, i32 0), i32 10, i32 %call)
ret i32 0
}