tail call generated by clang 1.1 and 1.0 (llvm 2.7 and 2.6)
- by ony
After compilation next snippet of code with clang -O2 (or with online demo):
#include <stdio.h>
#include <stdlib.h>
int flop(int x);
int flip(int x) {
if (x == 0) return 1;
return (x+1)*flop(x-1);
}
int flop(int x) {
if (x == 0) return 1;
return (x+0)*flip(x-1);
}
int main(int argc, char **argv) {
printf("%d\n", flip(atoi(argv[1])));
}
I'm getting next snippet of llvm assembly in flip:
bb1.i: ; preds = %bb1
%4 = add nsw i32 %x, -2 ; <i32> [#uses=1]
%5 = tail call i32 @flip(i32 %4) nounwind ; <i32> [#uses=1]
%6 = mul nsw i32 %5, %2 ; <i32> [#uses=1]
br label %flop.exit
I thought that tail call means dropping current stack (i.e. return will be to the upper frame, so next instruction should be ret %5), but according to this code it will do mul for it. And in native assembly there is simple call without tail optimisation (even with appropriate flag for llc)
Can sombody explain why clang generates such code?
As well I can't understand why llvm have tail call if it can simply check that next ret will use result of prev call and later do appropriate optimisation or generate native equivalent of tail-call instruction?
