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Nicholas White

2015-May-03 22:26 UTC

[LLVMdev] Semantics of an Inbounds GetElementPtr

Hi - I've got a question about what optimizations the "inbounds"
keyword of "getelementptr" allows you to use. In the code below, %five
is loaded from and inbounds offset of either a null pointer or %mem:

target datalayout = "e-i64:64-f80:128-n8:16:32:64-S128"

define i8 @func(i8* %mem) {
  %test = icmp eq i8* %mem, null
  br i1 %test, label %done, label %mem.is.valid
mem.is.valid:
  %1 = getelementptr inbounds i8, i8* %mem, i64 4
  store i8 5, i8* %1, align 4
  br label %done
done:
  %after.phi = phi i8* [ %mem, %mem.is.valid ], [ null, %0 ]
  %2 = getelementptr inbounds i8, i8* %after.phi, i64 4
  %five = load i8, i8* %2, align 4
  ret i8 %five
}

According to the documentation, "the result value of the getelementptr
is a poison value if the base pointer is not an in bounds address of
an allocated object", so does this mean it's valid to optimise the
function to:

define i8 @func(i8* %mem) {
  %test = icmp eq i8* %mem, null
  br i1 %test, label %done, label %mem.is.valid
mem.is.valid:
  ret i8 5
done:
  ret i8 undef
}

Or even this:

define i8 @func(i8* %mem) {
  ret i8 5
}

...? This is a reduced example of something I saw while running "opt"
on a test case that missed a null check. Thanks -

Nick

David Majnemer

2015-May-04 01:57 UTC

head link

[LLVMdev] Semantics of an Inbounds GetElementPtr

On Sun, May 3, 2015 at 6:26 PM, Nicholas White <n.j.white at gmail.com>
wrote:
> Hi - I've got a question about what optimizations the
"inbounds"
> keyword of "getelementptr" allows you to use. In the code below,
%five
> is loaded from and inbounds offset of either a null pointer or %mem:
>
> target datalayout = "e-i64:64-f80:128-n8:16:32:64-S128"
>
> define i8 @func(i8* %mem) {
>   %test = icmp eq i8* %mem, null
>   br i1 %test, label %done, label %mem.is.valid
> mem.is.valid:
>   %1 = getelementptr inbounds i8, i8* %mem, i64 4
>   store i8 5, i8* %1, align 4
>   br label %done
> done:
>   %after.phi = phi i8* [ %mem, %mem.is.valid ], [ null, %0 ]
>   %2 = getelementptr inbounds i8, i8* %after.phi, i64 4
>   %five = load i8, i8* %2, align 4
>   ret i8 %five
> }
>
> According to the documentation, "the result value of the getelementptr
> is a poison value if the base pointer is not an in bounds address of
> an allocated object", so does this mean it's valid to optimise the
> function to:
>
> define i8 @func(i8* %mem) {
>   %test = icmp eq i8* %mem, null
>   br i1 %test, label %done, label %mem.is.valid
> mem.is.valid:
>   ret i8 5
> done:
>   ret i8 undef
> }
>
> Or even this:
>
> define i8 @func(i8* %mem) {
>   ret i8 5
> }
>
No, neither are semantics preserving because there exists no store to
'%mem'.

Let's start by hoisting both sides of the branch into the entry block.
This will leave you with:
define i8 @func(i8* %mem) {
  %test = icmp eq i8* %mem, null
  %after.phi = select i1 %test, i8* null, %mem
  %1 = getelementptr inbounds i8, i8* %mem, i64 4
  store i8 5, i8* %1, align 4
  %2 = getelementptr inbounds i8, i8* %after.phi, i64 4
  %five = load i8, i8* %2, align 4
  ret i8 %five
}

The SSA value '%after.phi' can be trivially simplified to
'%mem', this
leaves us with:
define i8 @func(i8* %mem) {
  %1 = getelementptr inbounds i8, i8* %mem, i64 4
  store i8 5, i8* %1, align 4
  %2 = getelementptr inbounds i8, i8* %mem, i64 4
  %five = load i8, i8* %2, align 4
  ret i8 %five
}

The SSA values '%1' and '%2' are equivalent, this leaves us
with:
define i8 @func(i8* %mem) {
  %1 = getelementptr inbounds i8, i8* %mem, i64 4
  store i8 5, i8* %1, align 4
  ret i8 5
}

I do not believe further simplification is possible.

> ...? This is a reduced example of something I saw while running
"opt"
> on a test case that missed a null check. Thanks -
>
> Nick
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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David Majnemer

2015-May-04 01:59 UTC

head link

[LLVMdev] Semantics of an Inbounds GetElementPtr

On Sun, May 3, 2015 at 9:57 PM, David Majnemer <david.majnemer at
gmail.com>
wrote:
>
>
> On Sun, May 3, 2015 at 6:26 PM, Nicholas White <n.j.white at
gmail.com>
> wrote:
>
>> Hi - I've got a question about what optimizations the
"inbounds"
>> keyword of "getelementptr" allows you to use. In the code
below, %five
>> is loaded from and inbounds offset of either a null pointer or %mem:
>>
>> target datalayout = "e-i64:64-f80:128-n8:16:32:64-S128"
>>
>> define i8 @func(i8* %mem) {
>>   %test = icmp eq i8* %mem, null
>>   br i1 %test, label %done, label %mem.is.valid
>> mem.is.valid:
>>   %1 = getelementptr inbounds i8, i8* %mem, i64 4
>>   store i8 5, i8* %1, align 4
>>   br label %done
>> done:
>>   %after.phi = phi i8* [ %mem, %mem.is.valid ], [ null, %0 ]
>>   %2 = getelementptr inbounds i8, i8* %after.phi, i64 4
>>   %five = load i8, i8* %2, align 4
>>   ret i8 %five
>> }
>>
>> According to the documentation, "the result value of the
getelementptr
>> is a poison value if the base pointer is not an in bounds address of
>> an allocated object", so does this mean it's valid to optimise
the
>> function to:
>>
>> define i8 @func(i8* %mem) {
>>   %test = icmp eq i8* %mem, null
>>   br i1 %test, label %done, label %mem.is.valid
>> mem.is.valid:
>>   ret i8 5
>> done:
>>   ret i8 undef
>> }
>>
>> Or even this:
>>
>> define i8 @func(i8* %mem) {
>>   ret i8 5
>> }
>>
>
> No, neither are semantics preserving because there exists no store to
> '%mem'.
>
> Let's start by hoisting both sides of the branch into the entry block.
> This will leave you with:
> define i8 @func(i8* %mem) {
>   %test = icmp eq i8* %mem, null
>   %after.phi = select i1 %test, i8* null, %mem
>   %1 = getelementptr inbounds i8, i8* %mem, i64 4
>   store i8 5, i8* %1, align 4
>   %2 = getelementptr inbounds i8, i8* %after.phi, i64 4
>   %five = load i8, i8* %2, align 4
>   ret i8 %five
> }
>
> The SSA value '%after.phi' can be trivially simplified to
'%mem', this
> leaves us with:
> define i8 @func(i8* %mem) {
>   %1 = getelementptr inbounds i8, i8* %mem, i64 4
>   store i8 5, i8* %1, align 4
>   %2 = getelementptr inbounds i8, i8* %mem, i64 4
>   %five = load i8, i8* %2, align 4
>   ret i8 %five
> }
>
> The SSA values '%1' and '%2' are equivalent, this leaves us
with:
> define i8 @func(i8* %mem) {
>   %1 = getelementptr inbounds i8, i8* %mem, i64 4
>   store i8 5, i8* %1, align 4
>   ret i8 5
> }
>
> I do not believe further simplification is possible.
>
One final point that I forgot to mention: the transformations that I
performed did not rely on the presence of 'inbounds'.

>
>
>> ...? This is a reduced example of something I saw while running
"opt"
>> on a test case that missed a null check. Thanks -
>>
>> Nick
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>
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