[LLVMdev] Telling the optimizer a value is always null at the start

[David Blaikie]

On Thu, Jul 10, 2014 at 11:29 AM, Philip Reames
<listmail at philipreames.com> wrote:
> Hm, I don't know of an explicit way in the IR to do this.  If anyone else
> does, feel free to chime in.
>
> One approach would be to add a branch at the beginning of the function to an
> unreachable block.  If you're testeh function started with:
> if( *ex != null) llvm_unreachable();
>
> You might get lucky and have the GVN or EarlyCSE propagate the null value.
> You're going to run into pass ordering problems here though.  I suspect we'd
> drop the unreachable block before GVN runs.

Yep - SimplifyCFG does a great job of ripping branches to unreachable
out pretty early on.

> I looked at something like this
> previously:
> http://www.philipreames.com/Blog/2014/02/15/tweaking-llvm-to-exploit-assumex/
>
> If you're willing to tolerate an extra branch at runtime, you could simply
> use a return in place of the unreachable.  That would definitely work (i.e.
> no pass ordering problem), but the compare and branch would be emitted at
> runtime.
>
> Are you willing to extend the optimizer?  If so, adding a bit of metadata
> and the rules to propagate it in the optimizer would be pretty straight
> forward.  If you need this to work from C code (rather than IR), you'd also
> need to pass the information through clang.
>
> Philip
>
>
>
>
>
> On 07/10/2014 06:06 AM, Carlo Kok wrote:
>>
>> How do I tell the optimizer that the (dereferenced) value of an i8**
>> parameter is NULL at the start so that it can eliminate the check?
>>
>> I have code like:
>>
>>       void test2(void** ex) {
>>         printf("go\n"); // does not change *ex
>>       }
>>
>>       void call2(void** ex);
>>
>>       void testeh(void** ex) {
>>         // I want to tell the optimizer *ex is null so it can eliminate
>> the
>> first if below if test2 is inlined:
>>         test2(ex)
>>         if (*ex) return;
>>         printf("done")
>>       }
>>
>> I tried with llvm.invariant.start/end but that doesn't seem to make any
>> difference, the icmp/br seems to stay. Can this be done with the current
>> optimizers?
>>
>> My usecase is exception handling via an out parameter. The contract is
>> that on entry ex always points to a stack position which value holds null.
>>
>>
>> ; ModuleID = 'test.ll'
>> target datalayout = "e-m:w-p:32:32-i64:64-f80:32-n8:16:32-S32"
>> target triple = "i686-pc-windows-msvc"
>>
>> @str1 = linkonce_odr unnamed_addr constant [3 x i8] c"go\00", align 1
>> @str2 = linkonce_odr unnamed_addr constant [5 x i8] c"done\00", align 1
>>
>> define void @inlineeh(i8** nocapture readnone %ex) {
>>     %1 = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([3
>> x
>> i8]* @str1, i32 0, i32 0))
>>     ret void
>> }
>>
>> declare i32 @printf(i8* nocapture readonly, ...) #0
>>
>> define void @testeh(i8** nocapture %ex) #0 {
>>     %1 = bitcast i8** %ex to i8*
>>     %2 = tail call {}* @llvm.invariant.start(i64 4, i8* %1)
>>     tail call void @llvm.invariant.end({}* %2, i64 4, i8* %1)
>>     %3 = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([3
>> x
>> i8]* @str1, i32 0, i32 0))
>>     %4 = load i8** %ex, align 4
>>     %5 = icmp eq i8* %4, null
>>     br i1 %5, label %6, label %8
>>
>> ; <label>:6                                       ; preds = %0
>>     %7 = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([5
>> x
>> i8]* @str2, i32 0, i32 0))
>>     br label %8
>>
>> ; <label>:8                                       ; preds = %0, %6
>>     ret void
>> }
>>
>> declare void @llvm.invariant.end({}*, i64, i8* nocapture)
>> declare {}* @llvm.invariant.start(i64, i8* nocapture)
>>
>>
>>
>>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev