How the pre and post increment work in c++? [closed] - c++

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i want to know if increment/decrement operator results vary from compiler to compiler or it is independent of the compiler. as I am getting different results in different compilers of c/c++ (tested in G++, TDM, gcc) for the same expression.

Answer 1

Let's look at the internal llvm assembly. Which isn't hard to do.
C++ code:
void preincrement() __attribute__((noinline));
void preincrement() {
volatile int x = 10;
volatile int y = ++x;
}
void postincrement() __attribute__((noinline));
void postincrement() {
volatile int x = 10;
volatile int y = x++;
}
int main()
{
preincrement();
postincrement();
}
Resultant LLVM Assembly (minus some leading and trailing text that isn't interesting):
; Function Attrs: noinline nounwind uwtable
define void #_Z12preincrementv() #0 {
%x = alloca i32, align 4
%y = alloca i32, align 4
store volatile i32 10, i32* %x, align 4
%1 = load volatile i32* %x, align 4
%2 = add nsw i32 %1, 1
store volatile i32 %2, i32* %x, align 4
store volatile i32 %2, i32* %y, align 4
ret void
}
; Function Attrs: noinline nounwind uwtable
define void #_Z13postincrementv() #0 {
%x = alloca i32, align 4
%y = alloca i32, align 4
store volatile i32 10, i32* %x, align 4
%1 = load volatile i32* %x, align 4
%2 = add nsw i32 %1, 1
store volatile i32 %2, i32* %x, align 4
store volatile i32 %1, i32* %y, align 4
ret void
}
; Function Attrs: nounwind uwtable
define i32 #main() #1 {
tail call void #_Z12preincrementv()
tail call void #_Z13postincrementv()
ret i32 0
}
We can see here pretty clearly the only difference: In the post increment version, we use the original value, instead of the upgraded value.
In both cases, we just use a traditional add opcode to do the real work.