[llvm-dev] Help with SROA throwing away no-alias information

[Neil Henning via llvm-dev]

Hey Jeroen,

Ok cool - once I can work out how to apply that patch successfully without
using ARC I'll give it a whirl (sidenote: oh how I lament not just being
able to use github to pull the PR!).

Cheers,
-Neil.

On Mon, Jan 20, 2020 at 8:09 AM Jeroen Dobbelaere <
Jeroen.Dobbelaere at synopsys.com> wrote:

> Hi Neil,
>
>
>
> This should be solved with the 'full restrict patches'
> https://reviews.llvm.org/D68484 : it treats %noalias arguments in a
> different
>
> way so that we keep more accurate information during SROA.
>
> A convenience single patch is available here:
> https://reviews.llvm.org/D69542
>
>
>
> Please let me know, if you happen to try it out, that it indeed solves
> your problem.
>
>
>
> Thanks,
>
>
>
> Jeroen Dobbelaere
>
>
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Neil
> Henning via llvm-dev
> *Sent:* Friday, January 17, 2020 17:22
> *To:* llvm-dev <llvm-dev at lists.llvm.org>
> *Subject:* [llvm-dev] Help with SROA throwing away no-alias information
>
>
>
> I'm having an issue where SROA will throw away no-alias information on
> some loads after inlining, because the loads are derived from a store to an
> alloca which can be removed after inlining.
>
>
>
> The pointers that were originally stored into the alloca do * not *have
> any aliasing information - the only context that allowed me to assert
> aliasing was that the inlined-function guaranteed it to be so.
>
>
>
> I know why SROA has done this - its just removing a store -> load on an
> alloca by forwarding the store, but is there anyway I can preserve the
> information that these pointers must no-alias?
>
>
>
> Here is the pre/post from when SROA is run and you can see the now missing
> information of no-alias:
>
>
>
> define dllexport void
> @"Burst.Compiler.IL.Tests.Aliasing.CheckInlinedFunctionPreservesAliasing(ref
> Burst.Compiler.IL.Tests.Aliasing.NoAliasWithContentsStruct
> s)_862A52BC559AC318"(%"Burst.Compiler.IL.Tests.Aliasing/NoAliasWithContentsStruct"*
> nocapture nonnull readonly %s) local_unnamed_addr #0 !ubaa. !1 {
> entry:
>   %var.0 = alloca %"Burst.Compiler.IL.Tests.Aliasing/NoAliasField", align 8
>   %0 = bitcast
> %"Burst.Compiler.IL.Tests.Aliasing/NoAliasWithContentsStruct"* %s to i64*
>   %1 = load i64, i64* %0, align 1
>   %2 = getelementptr
> %"Burst.Compiler.IL.Tests.Aliasing/NoAliasWithContentsStruct",
> %"Burst.Compiler.IL.Tests.Aliasing/NoAliasWithContentsStruct"* %s, i64 0,
> i32 1
>   %3 = bitcast i8** %2 to i64*
>   %4 = load i64, i64* %3, align 1
>   %5 = bitcast %"Burst.Compiler.IL.Tests.Aliasing/NoAliasField"* %var.0 to
> i64*
>   store i64 %1, i64* %5, align 8
>   %.fca.1.gep = getelementptr inbounds
> %"Burst.Compiler.IL.Tests.Aliasing/NoAliasField",
> %"Burst.Compiler.IL.Tests.Aliasing/NoAliasField"* %var.0, i64 0, i32 1
>   %6 = bitcast i32** %.fca.1.gep to i64*
>   store i64 %4, i64* %6, align 8
>   %7 = bitcast %"Burst.Compiler.IL.Tests.Aliasing/NoAliasField"* %var.0 to
> i8**
>   %8 = load i8*, i8** %7, align 1, !alias.scope !2, !noalias !5
>   %9 = getelementptr %"Burst.Compiler.IL.Tests.Aliasing/NoAliasField",
> %"Burst.Compiler.IL.Tests.Aliasing/NoAliasField"* %var.0, i64 0, i32 1
>   %10 = bitcast i32** %9 to i8**
>   %11 = load i8*, i8** %10, align 1, !alias.scope !5, !noalias !2
>   call void @llvm.ubaa.expect.no.alias(i8* %8, i64 -1, i8* %11, i64 -1,
> metadata
> !"C:\\Users\\NeilHenning\\Home\\burst\\src\\Burst.Compiler.IL.Tests\\Shared\\083-Aliasing.cs(229,9):
> ") #0
>   ret void
> }
> *** IR Dump After SROA ***
> ; Function Attrs: nounwind
> define dllexport void
> @"Burst.Compiler.IL.Tests.Aliasing.CheckInlinedFunctionPreservesAliasing(ref
> Burst.Compiler.IL.Tests.Aliasing.NoAliasWithContentsStruct
> s)_862A52BC559AC318"(%"Burst.Compiler.IL.Tests.Aliasing/NoAliasWithContentsStruct"*
> nocapture nonnull readonly %s) local_unnamed_addr #0 !ubaa. !1 {
> entry:
>   %0 = bitcast
> %"Burst.Compiler.IL.Tests.Aliasing/NoAliasWithContentsStruct"* %s to i64*
>   %1 = load i64, i64* %0, align 1
>   %2 = getelementptr
> %"Burst.Compiler.IL.Tests.Aliasing/NoAliasWithContentsStruct",
> %"Burst.Compiler.IL.Tests.Aliasing/NoAliasWithContentsStruct"* %s, i64 0,
> i32 1
>   %3 = bitcast i8** %2 to i64*
>   %4 = load i64, i64* %3, align 1
>   %5 = inttoptr i64 %1 to i8*
>   %6 = inttoptr i64 %4 to i8*
>   call void @llvm.ubaa.expect.no.alias(i8* %5, i64 -1, i8* %6, i64 -1,
> metadata
> !"C:\\Users\\NeilHenning\\Home\\burst\\src\\Burst.Compiler.IL.Tests\\Shared\\083-Aliasing.cs(229,9):
> ") #0
>   ret void
> }
>
>
>
> My only workaround I can dream up locally is to run a pass before SROA to
> do the alias.scope and noalias forwarding myself by looking back through
> the store...
>
>
>
> Cheers,
>
> -Neil.
>
>
> --
>
> [image: Image removed by sender.]
>
> *Neil Henning*
>
> Senior Software Engineer Compiler
>
> unity.com
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__unity.com&d=DwMFaQ&c=DPL6_X_6JkXFx7AXWqB0tg&r=ELyOnT0WepII6UnFk-OSzxlGOXXSfAvOLT6E8iPwwJk&m=GSznZd2GUUZ6h5fLXYaTzhCHJwxEUo_GmtRYcMlOqPE&s=LVQA9YOzUMXrvsXSngTfQjveFSvNuRSG_C0S27GAu1w&e=>
>
>
>


-- 
Neil Henning
Senior Software Engineer Compiler
unity.com
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