[LLVMdev] clang -O is removing statements
Kito Cheng
kito.cheng at gmail.com
Sun Nov 9 22:10:05 PST 2014
Hi Jiang:
Try opt -sroa, I think that is what you want.
For example:
$ cat fib.c
int fib(int n){
int a = 10 - n; // Dead statement
if (n <= 1)
return 1;
else
return fib(n-1) + fib(n-2);
}
$ clang -emit-llvm fib.c -o fib.bc -S
$ opt fib.bc -sroa -S -o fib.sroa.bc
$ cat fib.sroa.bc
...
define i32 @fib(i32 %n) #0 {
%1 = sub nsw i32 10, %n ; Dead statement
%2 = icmp sle i32 %n, 1
br i1 %2, label %3, label %4
; <label>:3 ; preds = %0
br label %10
; <label>:4 ; preds = %0
%5 = sub nsw i32 %n, 1
%6 = call i32 @fib(i32 %5)
%7 = sub nsw i32 %n, 2
%8 = call i32 @fib(i32 %7)
%9 = add nsw i32 %6, %8
br label %10
; <label>:10 ; preds = %4, %3
%.0 = phi i32 [ 1, %3 ], [ %9, %4 ]
ret i32 %.0
}
...
On Mon, Nov 10, 2014 at 9:09 AM, Jiang LONG <jiang.long at berkeley.edu> wrote:
> Hi,
>
> I am using clang (latest) to compile C code and process the byte code.
> However, if I use -O to optimize, it is removing statements for some reason.
> I am wondering if there is a way to do the optimization of (alloca and
> store) but still keep all the statements un-touched.
>
> -Jiang
>
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