[LLVMdev] missed optimizations

[Nuno Lopes]

Hi,

I have two questions about optimizations performed by llvm.

Consider these simple functions:
int x(int b) { return b?4:6; }
int y() { return x(0); }

int x2() { return 5; }
int y2() { return x2(); }

the optimized bitcode (with clang + opt -std-compiler-opts) is:
define i32 @y(...) nounwind {
entry:
 ret i32 6
}

define i32 @y2(...) nounwind {
entry:
 %call = call i32 (...)* @x2( )  ; <i32> [#uses=0]
 ret i32 5
}

So why does LLVM optimizes the more difficult case, but leaves behind the 
function call in the easiest case? :)


Second question:
int f();

int g() {
 if (f() == 5)  return 3;
 return 4;
}

int h() {
 if (g() == 6) return 2;
 return 1;
}

gives the following optimized bc:

define i32 @g(...) nounwind {
entry:
 %call = call i32 (...)* @f( )  ; <i32> [#uses=1]
 %cmp = icmp eq i32 %call, 5  ; <i1> [#uses=1]
 %retval = select i1 %cmp, i32 3, i32 4  ; <i32> [#uses=1]
 ret i32 %retval
}

define i32 @h(...) nounwind {
entry:
 %call = call i32 (...)* @g( )  ; <i32> [#uses=1]
 %cmp = icmp eq i32 %call, 6  ; <i1> [#uses=1]
 %retval = select i1 %cmp, i32 2, i32 1  ; <i32> [#uses=1]
 ret i32 %retval
}

In function h(), llvm doesn't realize that g() never returns 6, and thus it 
could reduce that function to { g(); return 1; }. Doesn't llvm produce some 
sort of summary for functions with e.g. their possible return values? If 
not, is there any pass where I could implement this?  (I have some code that 
has many opportunities for this kind of optimization)
Also, shouldn't the function g() get inlined in the h() function? It is 
inlined only if I change the g() function to be static. So isn't llvm being 
too conservative when inlining global functions?

Thanks,
Nuno