This is my bitcode compiled using llvm:
define void #.datadiv_decode2604521171261794706() {
; <label>:0:
br label %1
br label %6
; <label>:1: ; preds = %1, %0
%2 = phi i32 [ 0, %0 ], [ %3, %1 ]
%3 = add i32 %2, 1
%4 = icmp ult i32 %2, 12
%5 = icmp ne i1 %4, i32 0
br i1 %5, label %1, label %0
; <label>:6: ; preds = %6, %0
%7 = phi i32 [ 0, %0 ], [ %8, %6 ]
%8 = add i32 %7, 1
%9 = icmp ult i32 %7, 2
%10 = icmp ne i1 %9, i32 0
br i1 %10, label %6, label %0
}
And error message is :
Error: instruction expected to be numbered '%3'.
What is the solution?
; <label>:0:
br label %1
br label %6
This is entirely wrong. Each BasicBlock should only have one terminator. Please provide the source of whatever Transform pass you've written to help locate your error.
Related
I notice that in LLVM bitcode files, all PHI instructions in the same basic block often have the same set of incoming blocks.
Does anyone know if it is always true for all LLVM bitcode files?
Or is there any optimization pass for doing so?
For example, here is my C code:
// test.c
int main(){
int **p, *q;
int *a, *b, c, d;
p = &a;
if (p) {
if (c) {
q = &c;
}
}
else{
p = &b;
q = &d;
}
if (d) {
*p = q;
}
}
After being compiled by clang and opt:
clang -Xclang -disable-O0-optnone -c -emit-llvm test.c
opt -mem2reg test.bc -o test.opt.bc
Here is the output test.opt.bc, where all PHI instructions in block 12 have the same incomming blocks 11 and 12:
; Function Attrs: noinline nounwind uwtable
define dso_local i32 #main() #0 {
%1 = alloca i32*, align 8
%2 = alloca i32*, align 8
%3 = alloca i32, align 4
%4 = alloca i32, align 4
%5 = icmp ne i32** %1, null
br i1 %5, label %6, label %11
6: ; preds = %0
%7 = load i32, i32* %3, align 4
%8 = icmp ne i32 %7, 0
br i1 %8, label %9, label %10
9: ; preds = %6
br label %10
10: ; preds = %9, %6
br label %12
11: ; preds = %0
br label %12
12: ; preds = %11, %10
%.01 = phi i32** [ %1, %10 ], [ %2, %11 ]
%.1 = phi i32* [ %3, %10 ], [ %4, %11 ]
%13 = load i32, i32* %4, align 4
%14 = icmp ne i32 %13, 0
br i1 %14, label %15, label %16
15: ; preds = %12
store i32* %.1, i32** %.01, align 8
br label %16
16: ; preds = %15, %12
ret i32 0
}
The answer is yes.
It directly follows from the phi instruction description from the language reference:
After this, the ‘phi’ instruction takes a list of pairs as arguments, with one pair for each predecessor basic block of the current block.
Since all of phi instructions in question belong to the same basic block, these lists of pairs should reference the same set of predecessors.
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've been playing around with compilers and have been working on my own toy C compiler. Currently I'm attempting to target LLVM IR, but I'm having trouble wrapping my head around the syntax.
My current current issue: why is this valid IR syntax:
define i32 #main() {
%1 = alloca i32, align 4
%2 = add i32 0, 0
store i32 %2, i32* %1, align 4
%3 = alloca i32, align 4
%4 = add i32 0, 1
store i32 %4, i32* %3, align 4
%5 = load i32, i32* %1, align 4
%6 = icmp ne i32 %5, 0
br i1 %6, label %true0, label %else0
true0: ; preds %0
%7 = add i32 0, 1
store i32 %7, i32* %3, align 4
br label %end0
else0: ; preds %0
%8 = load i32, i32* %3, align 4
%9 = icmp ne i32 %8, 0
br i1 %9, label %true1, label %end1
true1: ; preds %else0
%10 = add i32 0, 2
store i32 %10, i32* %3, align 4
br label %end1
end1: ; preds %true1, %else0
br label %end0
end0: ; preds %true0, %else1
%11 = load i32, i32* %3, align 4
ret i32 %11
}
but this is not:
define i32 #main() {
%1 = alloca i32, align 4
%2 = add i32 0, 0
store i32 %2, i32* %1, align 4 ; variable a
%3 = load i32, i32* %1, align 4
%4 = icmp ne i32 %3, 0
br i1 %4, label %true0, label %else0
true0: ; preds %0
%5 = add i32 0, 1
ret i32 %5
br label %end0
else0: ; preds %0
%6 = add i32 0, 2
ret i32 %6
br label %end0
end0: ; % preds %true0, %else0
ret i32 0
}
I get the error:
llc-6.0: test2.ll:13:1: error: instruction expected to be numbered '%7'
%6 = add i32 0, 2
^
I don't understand why that block needs to be %7, given the previously used number was %6. Compare the %else0 label of the first example, that's very similar syntax and works fine.
And yes, my compiler needs a lot of optimization, but I'm not finished yet :)
Your code is invalid because there is actually another basic block you did not labeled:
true0: ; preds %0
%5 = add i32 0, 1
ret i32 %5
hidden_bb: ; this will named as %6 by default
br label %end0
else0: ; preds %0
If it has a label than the error will gone. Note that all terminator instructions, like br and ret will create their own basic block.
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).
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)