Skip to content

[DAGCombiner] Ensure poison-generating flags are stripped in freeze op #114582

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Closed
wants to merge 4 commits into from
Closed

[DAGCombiner] Ensure poison-generating flags are stripped in freeze op #114582

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
4 commits
Select commit Hold shift + click to select a range
af4c171
[DAGCombiner] Ensure poison-generating flags are stripped in `freeze` op
antoniofrighetto Nov 1, 2024
f731a72
!fixup drop dead attribute
antoniofrighetto Nov 1, 2024
0c5e0a2
!fixup drop -mattr
antoniofrighetto Nov 1, 2024
1c5a0fc
!fixup simplify test
antoniofrighetto Nov 1, 2024
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
1 change: 1 addition & 0 deletions llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -16151,6 +16151,7 @@ SDValue DAGCombiner::visitFREEZE(SDNode *N) {
SVN->getMask());
} else {
// NOTE: this strips poison generating flags.
N0->dropFlags(SDNodeFlags::PoisonGeneratingFlags);
Copy link
Member

@dtcxzyw dtcxzyw Nov 2, 2024 •
edited
Loading

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I don't think it is a correct fix. The following getNode creates a new copy without flags. However, it is CSEed to N0 in SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, SDVTList VTList, ArrayRef<SDValue> Ops, const SDNodeFlags Flags).

We should intersect flags after CSE here:

llvm-project/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

Lines 10321 to 10322 in 92daad2

if (SDNode *E = FindNodeOrInsertPos(ID, DL, IP))
return SDValue(E, 0);

llvm-project/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

Lines 10527 to 10528 in 92daad2

if (SDNode *E = FindNodeOrInsertPos(ID, DL, IP))
return SDValue(E, 0);

I will post a fix later.

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

getNode creates a copy, but N0 is still the original node. It is indeed CSE'd, but the flag has been dropped by the time it gets CSE'd.

Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

but the flag has been dropped by the time it gets CSE'd.

I printed the node returned by FindNodeOrInsertPos and got select_cc samesign. The flags are not dropped. It should be fixed by #114650.

Copy link
Contributor Author

@antoniofrighetto antoniofrighetto Nov 2, 2024 •
edited
Loading

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Quite strange. I just tried again and confirm the node returned here:

llvm-project/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

Lines 10321 to 10322 in 89a8c71

if (SDNode *E = FindNodeOrInsertPos(ID, DL, IP))
return SDValue(E, 0);

is i32 = select_cc t12, t28, t16, t2, setult:ch, immediately after visiting freeze in DAGCombiner, nullptr otherwise. I'm happy with your patch too.

Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

baseline: f1e1055

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 7eef09e55101..b5bc6d925b41 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -16150,8 +16150,12 @@ SDValue DAGCombiner::visitFREEZE(SDNode *N) {
     R = DAG.getVectorShuffle(N0.getValueType(), SDLoc(N0), Ops[0], Ops[1],
                              SVN->getMask());
   } else {
+    errs() << "N0: ";
+    N0->dump();
     // NOTE: this strips poison generating flags.
     R = DAG.getNode(N0.getOpcode(), SDLoc(N0), N0->getVTList(), Ops);
+    errs() << "R: ";
+    R->dump();
   }
   assert(DAG.isGuaranteedNotToBeUndefOrPoison(R, /*PoisonOnly*/ false) &&
          "Can't create node that may be undef/poison!");
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index d5cdd7163d79..a533ac910588 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -10318,8 +10318,11 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
     AddNodeIDNode(ID, Opcode, VTs, Ops);
     void *IP = nullptr;
 
-    if (SDNode *E = FindNodeOrInsertPos(ID, DL, IP))
+    if (SDNode *E = FindNodeOrInsertPos(ID, DL, IP)) {
+      errs() << "E: ";
+      E->dump();
       return SDValue(E, 0);
+    }
 
     N = newSDNode<SDNode>(Opcode, DL.getIROrder(), DL.getDebugLoc(), VTs);
     createOperands(N, Ops);

N0: t27: i32 = select_cc samesign t12, t28, t16, t2, setult:ch
E: t27: i32 = select_cc samesign t12, t28, t16, t2, setult:ch
R: t27: i32 = select_cc samesign t12, t28, t16, t2, setult:ch

R = DAG.getNode(N0.getOpcode(), SDLoc(N0), N0->getVTList(), Ops);
}
assert(DAG.isGuaranteedNotToBeUndefOrPoison(R, /*PoisonOnly*/ false) &&
Expand Down
25 changes: 25 additions & 0 deletions llvm/test/CodeGen/ARM/dagcombine-drop-flags-freeze.ll
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,25 @@
; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
; RUN: llc -mtriple=arm-linux-gnueabi -mcpu=arm1022e -o - %s | FileCheck %s

; Ensure poison-generating flags are stripped by the time a freeze operand is visited.

define i1 @drop_flags(i32 noundef %numentries, i64 %cond, i64 %arg) {
; CHECK-LABEL: drop_flags:
; CHECK: @ %bb.0: @ %entry
; CHECK-NEXT: ldm sp, {r1, r12}
; CHECK-NEXT: subs r1, r2, r1
; CHECK-NEXT: sbcs r1, r3, r12
; CHECK-NEXT: movlo r0, r2
; CHECK-NEXT: clz r0, r0
; CHECK-NEXT: lsr r0, r0, #5
; CHECK-NEXT: bx lr
entry:
%cmp4 = icmp samesign ult i64 %cond, %arg
%conv6 = trunc nuw i64 %cond to i32
%spec.select = select i1 %cmp4, i32 %conv6, i32 %numentries
%spec.select.fr = freeze i32 %spec.select
%cmpz = icmp eq i32 %spec.select.fr, 0
ret i1 %cmpz
}

declare i32 @llvm.ctlz.i32(i32, i1)
Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Unused.

Loading