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[DSE] Look through memory PHI arguments when removing noop stores in MSSA.

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Summary:
Adds support for "following" memory through MSSA PHI arguments. This will help catch more noop stores that exist between blocks.

Originally part of D79391.

Reviewers: fhahn, jfb, asbirlea

Differential Revision: https://reviews.llvm.org/D82588
This commit is contained in:
zoecarver 2020-10-01 10:40:03 -07:00
parent 37e405d0af
commit b38d4083e5
3 changed files with 128 additions and 30 deletions
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40
lib/Transforms/Scalar/DeadStoreElimination.cpp
View File

@ -2404,10 +2404,44 @@ struct DSEState {
if (auto *LoadI = dyn_cast<LoadInst>(Store->getOperand(0))) {
if (LoadI->getPointerOperand() == Store->getOperand(1)) {
// Get the defining access for the load.
auto *LoadAccess = MSSA.getMemoryAccess(LoadI)->getDefiningAccess();
// If both accesses share the same defining access, no instructions
// between them can modify the memory location.
return LoadAccess == Def->getDefiningAccess();
// Fast path: the defining accesses are the same.
if (LoadAccess == Def->getDefiningAccess())
return true;
// Look through phi accesses. Recursively scan all phi accesses by
// adding them to a worklist. Bail when we run into a memory def that
// does not match LoadAccess.
SetVector<MemoryAccess *> ToCheck;
MemoryAccess *Current = Def->getDefiningAccess();
// We don't want to bail when we run into the store memory def. But,
// the phi access may point to it. So, pretend like we've already
// checked it.
ToCheck.insert(Def);
ToCheck.insert(Current);
// Start at current (1) to simulate already having checked Def.
for (unsigned I = 1; I < ToCheck.size(); ++I) {
Current = ToCheck[I];
if (auto PhiAccess = dyn_cast<MemoryPhi>(Current)) {
// Check all the operands.
for (auto &Use : PhiAccess->incoming_values())
ToCheck.insert(cast<MemoryAccess>(&Use));
continue;
}
// If we found a memory def, bail. This happens when we have an
// unrelated write in between an otherwise noop store.
assert(isa<MemoryDef>(Current) &&
"Only MemoryDefs should reach here.");
// TODO: Skip no alias MemoryDefs that have no aliasing reads.
// We are searching for the definition of the store's destination.
// So, if that is the same definition as the load, then this is a
// noop. Otherwise, fail.
if (LoadAccess != Current)
return false;
}
return true;
}
}

93
test/Transforms/DeadStoreElimination/MSSA/noop-stores.ll
View File

@ -101,6 +101,47 @@ bb3:
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 [[C:%.*]], 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 %c, label %bb1, label %bb2
bb2:
ret i32 0
}
; Don't remove "redundant" store if %p is possibly stored to.
define i32 @test46(i1 %c, i32* %p, i32* %p2, i32 %i) {
; CHECK-LABEL: @test46(
; CHECK: load
; CHECK: store
; CHECK: store
; CHECK: ret i32 0
;
entry:
%v = load i32, i32* %p, align 4
br label %bb1
bb1:
store i32 %v, i32* %p, align 4
br i1 %c, label %bb1, label %bb2
bb2:
store i32 0, i32* %p2, align 4
br i1 %c, label %bb3, label %bb1
bb3:
ret i32 0
}
declare void @unknown_func()
; Remove redundant store, which is in the lame loop as the load.
@ -112,7 +153,7 @@ define i32 @test33(i1 %c, i32* %p, i32 %i) {
; CHECK-NEXT: br label [[BB2:%.*]]
; CHECK: bb2:
; CHECK-NEXT: call void @unknown_func()
; CHECK-NEXT: br i1 undef, label [[BB1]], label [[BB3:%.*]]
; CHECK-NEXT: br i1 [[C:%.*]], label [[BB1]], label [[BB3:%.*]]
; CHECK: bb3:
; CHECK-NEXT: ret i32 0
;
@ -125,7 +166,7 @@ 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
br i1 %c, label %bb1, label %bb3
bb3:
ret i32 0
}
@ -168,4 +209,52 @@ define void @test45(i32* %Q) {
ret void
}
define i32 @test48(i1 %c, i32* %p) {
; CHECK-LABEL: @test48(
; CHECK: entry:
; CHECK-NEXT: [[V:%.*]] = load
; CHECK: store i32 0
; CHECK: store i32 [[V]]
; CHECK: ret i32 0
entry:
%v = load i32, i32* %p, align 4
br i1 %c, label %bb0, label %bb0.0
bb0:
store i32 0, i32* %p
br i1 %c, label %bb1, label %bb2
bb0.0:
br label %bb1
bb1:
store i32 %v, i32* %p, align 4
br i1 %c, label %bb2, label %bb0
bb2:
ret i32 0
}
; TODO: Remove both redundant stores if loaded value is in another block inside a loop.
define i32 @test47(i1 %c, i32* %p, i32 %i) {
; X-CHECK-LABEL: @test47(
; X-CHECK-NEXT: entry:
; X-CHECK-NEXT: br label [[BB1:%.*]]
; X-CHECK: bb1:
; X-CHECK-NEXT: br i1 [[C:%.*]], label [[BB1]], label [[BB2:%.*]]
; X-CHECK: bb2:
; X-CHECK-NEXT: br i1 [[C]], label [[BB2]], label [[BB3:%.*]]
; X-CHECK: bb3:
; X-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 %c, label %bb1, label %bb2
bb2:
store i32 %v, i32* %p, align 4
br i1 %c, label %bb3, label %bb1
bb3:
ret i32 0
}

25
test/Transforms/DeadStoreElimination/MSSA/simple-todo.ll
View File

@ -1,25 +0,0 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; XFAIL: *
; RUN: opt < %s -basic-aa -dse -enable-dse-memoryssa -S | FileCheck %s
; RUN: opt < %s -aa-pipeline=basic-aa -passes=dse -enable-dse-memoryssa -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"
; 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
}