[LLVMdev] simple optimization question

Duncan Sands baldrick at free.fr
Fri Jan 18 04:19:35 PST 2013

Hi Bjorn,

> currently the following two lines of code with integer variables get compiled completely differently:
> if (d0<tmp) d0=tmp;

there must be something else going on because this

void bar(int);
void foo(int d0, int tmp) {
   if (d0 < tmp) d0=tmp;

when compiled like this

clang -S -O4 -o - sel.c

gives this

define void @foo(i32 %d0, i32 %tmp) nounwind uwtable {
   %cmp = icmp slt i32 %d0, %tmp
   %tmp.d0 = select i1 %cmp, i32 %tmp, i32 %d0
   tail call void @bar(i32 %tmp.d0) nounwind
   ret void

Did you supply the optimizers with DataLayout information.  I mean this kind of
stuff in the IR:

target datalayout = 

Ciao, Duncan.

> d0 = d0<tmp? tmp : do;
> The first is compiled into two separate basic blocks, which my backend then optimizes into a conditional move with if-conversion (after instruction selection so not much optimization possible anymore). The second line is compiled into select operations, which my backend is able to compile into much more efficient max instructions.
> Shouldn't the first form be optimized into the second somewhere in the compiler front-end? In other words, do we already have a pass that should optimize this, and if not, is it worthwhile working on one?
> Thanks,
> Bjorn De Sutter
> Computer Systems Lab
> Ghent University
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