[llvm-bugs] [Bug 45795] New: [DSE, MSSA] Implement eliminating 'noop' stores

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45795

            Bug ID: 45795
           Summary: [DSE,MSSA] Implement eliminating 'noop' stores
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: florian_hahn at apple.com
                CC: llvm-bugs at lists.llvm.org

The MemorySSA-backed DSE does not yet support eliminating redundant 'noop'
stores, where the store stores a value just loaded from the same address like
in the examples below:

; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt < %s -basicaa -dse -S | FileCheck %s
; RUN: opt < %s -aa-pipeline=basic-aa -passes=dse -S | FileCheck %s
target datalayout =
"E-p:64:64:64-a0:0:8-f32:32:32-f64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-v64:64:64-v128:128:128"

define void @test5(i32* %Q) {
; CHECK-LABEL: @test5(
; CHECK-NEXT:    [[A:%.*]] = load volatile i32, i32* [[Q:%.*]]
; CHECK-NEXT:    ret void
;
  %a = load volatile i32, i32* %Q
  store i32 %a, i32* %Q
  ret void
}

; PR2599 - load -> store to same address.
define void @test12({ i32, i32 }* %x) nounwind  {
; CHECK-LABEL: @test12(
; CHECK-NEXT:    [[TMP7:%.*]] = getelementptr { i32, i32 }, { i32, i32 }*
[[X:%.*]], i32 0, i32 1
; CHECK-NEXT:    [[TMP8:%.*]] = load i32, i32* [[TMP7]], align 4
; CHECK-NEXT:    [[TMP17:%.*]] = sub i32 0, [[TMP8]]
; CHECK-NEXT:    store i32 [[TMP17]], i32* [[TMP7]], align 4
; CHECK-NEXT:    ret void
;
  %tmp4 = getelementptr { i32, i32 }, { i32, i32 }* %x, i32 0, i32 0
  %tmp5 = load i32, i32* %tmp4, align 4
  %tmp7 = getelementptr { i32, i32 }, { i32, i32 }* %x, i32 0, i32 1
  %tmp8 = load i32, i32* %tmp7, align 4
  %tmp17 = sub i32 0, %tmp8
  store i32 %tmp5, i32* %tmp4, align 4
  store i32 %tmp17, i32* %tmp7, align 4
  ret void
}

; Remove redundant store if loaded value is in another block.
define i32 @test26(i1 %c, i32* %p) {
; CHECK-LABEL: @test26(
; CHECK-NEXT:  entry:
; CHECK-NEXT:    br i1 [[C:%.*]], label [[BB1:%.*]], label [[BB2:%.*]]
; CHECK:       bb1:
; CHECK-NEXT:    br label [[BB3:%.*]]
; CHECK:       bb2:
; CHECK-NEXT:    br label [[BB3]]
; CHECK:       bb3:
; CHECK-NEXT:    ret i32 0
;
entry:
  %v = load i32, i32* %p, align 4
  br i1 %c, label %bb1, label %bb2
bb1:
  br label %bb3
bb2:
  store i32 %v, i32* %p, align 4
  br label %bb3
bb3:
  ret i32 0
}

; Remove redundant store if loaded value is in another block.
define i32 @test27(i1 %c, i32* %p) {
; CHECK-LABEL: @test27(
; CHECK-NEXT:  entry:
; CHECK-NEXT:    br i1 [[C:%.*]], label [[BB1:%.*]], label [[BB2:%.*]]
; CHECK:       bb1:
; CHECK-NEXT:    br label [[BB3:%.*]]
; CHECK:       bb2:
; CHECK-NEXT:    br label [[BB3]]
; CHECK:       bb3:
; CHECK-NEXT:    ret i32 0
;
entry:
  %v = load i32, i32* %p, align 4
  br i1 %c, label %bb1, label %bb2
bb1:
  br label %bb3
bb2:
  br label %bb3
bb3:
  store i32 %v, i32* %p, align 4
  ret i32 0
}

; Remove redundant store if loaded value is in another block inside a loop.
define i32 @test31(i1 %c, i32* %p, i32 %i) {
; CHECK-LABEL: @test31(
; CHECK-NEXT:  entry:
; CHECK-NEXT:    br label [[BB1:%.*]]
; CHECK:       bb1:
; CHECK-NEXT:    br i1 undef, label [[BB1]], label [[BB2:%.*]]
; CHECK:       bb2:
; CHECK-NEXT:    ret i32 0
;
entry:
  %v = load i32, i32* %p, align 4
  br label %bb1
bb1:
  store i32 %v, i32* %p, align 4
  br i1 undef, label %bb1, label %bb2
bb2:
  ret i32 0
}

; Remove redundant store, which is in the lame loop as the load.
define i32 @test33(i1 %c, i32* %p, i32 %i) {
; CHECK-LABEL: @test33(
; CHECK-NEXT:  entry:
; CHECK-NEXT:    br label [[BB1:%.*]]
; CHECK:       bb1:
; CHECK-NEXT:    br label [[BB2:%.*]]
; CHECK:       bb2:
; CHECK-NEXT:    call void @unknown_func()
; CHECK-NEXT:    br i1 undef, label [[BB1]], label [[BB3:%.*]]
; CHECK:       bb3:
; CHECK-NEXT:    ret i32 0
;
entry:
  br label %bb1
bb1:
  %v = load i32, i32* %p, align 4
  br label %bb2
bb2:
  store i32 %v, i32* %p, align 4
  ; Might read and overwrite value at %p, but doesn't matter.
  call void @unknown_func()
  br i1 undef, label %bb1, label %bb3
bb3:
  ret i32 0
}

declare void @unknown_func()

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