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[memcpyopt] fails to eliminate memcpy fed by sret #2590

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llvmbot opened this issue Apr 11, 2008 · 3 comments
Closed

[memcpyopt] fails to eliminate memcpy fed by sret #2590

llvmbot opened this issue Apr 11, 2008 · 3 comments
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bugzilla Issues migrated from bugzilla llvm:optimizations

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@llvmbot
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llvmbot commented Apr 11, 2008

Bugzilla Link 2218
Version unspecified
OS Linux
Attachments testcase .ll
Reporter LLVM Bugzilla Contributor
CC @efriedma-quic

Extended Description

Here's the testcase (also attached). %pointless could be
eliminated since no use is ever made of it except as a
memcpy target. %temporary could also be eliminated in
favour of passing %result directly to the @​initialize call.

define void @​f(i8* noalias sret %result) {
entry:
%temporary = alloca i8 ; <i8*> [#uses=3]
%pointless = alloca i8 ; <i8*> [#uses=1]
call void @​initialize( i8* noalias sret %temporary )
call void @​llvm.memcpy.i32( i8* %pointless, i8* %temporary, i32 1, i32 4 )
call void @​llvm.memcpy.i32( i8* %result, i8* %temporary, i32 1, i32 4 )
ret void
}

declare void @​initialize(i8* noalias sret )
declare void @​llvm.memcpy.i32(i8*, i8*, i32, i32) nounwind
@efriedma-quic
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Running the given testcase through opt -dse -memcpyopt -dce does the desired reductions, but opt -std-compile-opts doesn't.

@lattner
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lattner commented Nov 18, 2010

This is two issues:

  1. memcpyopt should delete memcpy's that store to otherwise dead allocas as general cleanup like DSE does.
  2. we don't optimize a memcpy fed by a sret aggressively, like:

define void @​f(i8* noalias sret %result) {
entry:
%temporary = alloca i8
call void @​initialize(i8* noalias sret %temporary)
call void @​llvm.memcpy.p0i8.p0i8.i32(i8* %result, i8* %temporary, i32 1, i32 4, i1 false)
ret void
}

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 3, 2021
cyndyishida pushed a commit to cyndyishida/llvm-project that referenced this issue Jun 17, 2022
@varungandhi-apple
Merge pull request llvm#2590 from varungandhi-apple/vg-rework-async
fd909ff 
Tweak async calling convention patches to reflect upstream changes
@arsenm
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arsenm commented Aug 4, 2023

opt -O2 through modernized testcase removes all the memcpies:

define void @f(ptr noalias sret(i8) %result) {
entry:
  %temporary = alloca i8, align 1
  %pointless = alloca i8, align 1
  call void @initialize(ptr noalias sret(i8) %temporary)
  call void @llvm.memcpy.p0.p0.i32(ptr align 4 %pointless, ptr align 4 %temporary, i32 1, i1 false)
  call void @llvm.memcpy.p0.p0.i32(ptr align 4 %result, ptr align 4 %temporary, i32 1, i1 false)
  ret void
}

declare void @initialize(ptr noalias sret(i8))

; Function Attrs: nocallback nofree nounwind willreturn memory(argmem: readwrite)
declare void @llvm.memcpy.p0.p0.i32(ptr noalias nocapture writeonly, ptr noalias nocapture readonly, i32, i1 immarg) #0

attributes #0 = { nocallback nofree nounwind willreturn memory(argmem: readwrite) }

@arsenm arsenm closed this as completed Aug 4, 2023
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@arsenm @EugeneZelenko @Endilll @lattner @efriedma-quic @llvmbot