[LLVMdev] code generation removes duplicated instructions

D S Khudia daya.khudia at gmail.com
Wed Jul 6 07:57:42 PDT 2011


The code snippet pasted in the previous email are generated at -O0 with llc.
Since I am inserting a new basic block (contains printf statement and
program exit) which is jumped upon based on the result of
the comparison, the compiler cannot/shouldnot optimize that away by means of
DCE or anything else.

The same kind of stuff is happening for the following duplication.

bb6.split:                                        ; preds = %bb6
  %33 = load i32* %32, align 4
  %34 = load i32* %i, align 4
  %HV4_3 = sub nsw i32 %34, %33
  %35 = sub nsw i32 %34, %33
  %HV4_2 = getelementptr inbounds [100 x i32]* %a, i32 0, i32 %HV4_3
  %36 = getelementptr inbounds [100 x i32]* %a, i32 0, i32 %35
  %LDCmp6 = icmp ne i32* %36, %HV4_2
  br i1 %LDCmp6, label %relExit, label %bb6.split.split

In the above example even the operands are not same and I guess compiler
cannot be that smart at -O0. I sense something is wrong with the code
generation for ARM.

What other way do you suggest for duplicating since you mentioned I
shouldn't rely on duplication the way I am doing it?

Thanks a lot. I really appreciate your help.

On Wed, Jul 6, 2011 at 10:27 AM, Renato Golin <renato.golin at arm.com> wrote:

> On 6 July 2011 14:55, D S Khudia <daya.khudia at gmail.com> wrote:
> > The following is an example code generation for arm and x86 for a same IR
> > BB. In the x86 code I can see that the same computation is done twice and
> > result is stored in two different registers and then these two different
> > registers are used for comparision.
> Yes, but you shouldn't rely on it, since the compiler is free to
> optimize that away.
> > By the way I am duplicating instruction
> > and inserting comparison to catch transient errors.
> I thought so. Try running llc with -O0 or disable specific
> optimizations (like dead-code elimination) to keep your comparisons
> intact.
> But since the two values are identical, it's possible that even so,
> both will live in the same register.
> cheers,
> --renato
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