More cases for flags

Agreement on POJ and GCJ

In this part, we introduce some flags that shows up in both POJ and GCJ.

-mem2reg

-mem2reg : It promotes alloca instructions which only have loads and stores as uses.

example source code:

int foo(int x, int cond)

{

if (cond > 0)

x = 1;

else

x = -1;

return x;

}

LLVM-IR code without -mem2reg

define dso_local i32 @foo(i32 %x, i32 %cond) #0 {

entry:

%x.addr = alloca i32, align 4

%cond.addr = alloca i32, align 4

store i32 %x, i32* %x.addr, align 4

store i32 %cond, i32* %cond.addr, align 4

%0 = load i32, i32* %cond.addr, align 4

%cmp = icmp sgt i32 %0, 0

br i1 %cmp, label %if.then, label %if.else

if.then: ; preds = %entry

store i32 1, i32* %x.addr, align 4

br label %if.end

if.else: ; preds = %entry

store i32 -1, i32* %x.addr, align 4

br label %if.end

if.end: ; preds = %if.else, %if.then

%1 = load i32, i32* %x.addr, align 4

ret i32 %1}

LLVM-IR code with -mem2reg

define dso_local i32 @foo(i32 %x, i32 %cond) #0 {

entry:

%cmp = icmp sgt i32 %cond, 0

br i1 %cmp, label %if.then, label %if.else

if.then: ; preds = %entry

br label %if.end

if.else: ; preds = %entry

br label %if.end

if.end: ; preds = %if.else, %if.then

%x.addr.0 = phi i32 [ 1, %if.then ], [ -1, %if.else ]

ret i32 %x.addr.0}

We can find that the load/store/alloca insts are all turned into real SSA-format, which helps reduce the useless inst and boost the quality of IR Code.

3) Simplify CFG.

-loop-rotate: converts loops into do/while style loops. The below example are from LLVM_LOOP_ROTATE

-instcombine

-instcombine :Combine instructions to form fewer, simple instructions.

example source code:

int main() {

int i =0;

int a = 1,b=2,c=3;

float k = 34;

i += a;

i += k;

printf("%d",i);

return i;

}

LLVM-IR code without -instcombine

define dso_local i32 @main() #0 {

%1 = alloca i32, align 4

%2 = alloca i32, align 4

%3 = alloca i32, align 4

%4 = alloca i32, align 4

%5 = alloca i32, align 4

%6 = alloca float, align 4

store i32 0, i32* %1, align 4

store i32 0, i32* %2, align 4

store i32 1, i32* %3, align 4

store i32 2, i32* %4, align 4

store i32 3, i32* %5, align 4

store float 3.400000e+01, float* %6, align 4

%7 = load i32, i32* %3, align 4

%8 = load i32, i32* %2, align 4

%9 = add nsw i32 %8, %7

store i32 %9, i32* %2, align 4

%10 = load float, float* %6, align 4

%11 = load i32, i32* %2, align 4

%12 = sitofp i32 %11 to float

%13 = fadd float %12, %10

%14 = fptosi float %13 to i32

store i32 %14, i32* %2, align 4

%15 = load i32, i32* %2, align 4

%16 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([3 x i8], [3 x i8]* @.str, i64 0, i64 0), i32 %15)

%17 = load i32, i32* %2, align 4

ret i32 %17

}

declare dso_local i32 @printf(i8*, ...) #1

LLVM-IR code with -instcombine

define dso_local i32 @main() #0 {

%1 = call i32 (i8*, ...) @printf(i8* nonnull dereferenceable(1) getelementptr inbounds ([3 x i8], [3 x i8]* @.str, i64 0, i64 0), i32 35) #2

ret i32 35

}

declare dso_local i32 @printf(i8*, ...) #1

As we can see, the abudunt instructions can be removed through -instcomine, we should also mention that different types can be analysed and convert into the same if not needed.