[llvm-branch-commits] [llvm] release/23.x: [MIPS] Fix GP-relative selection after select/add combine (#215414) (PR #220795)
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Wed Sep 2 21:02:24 PDT 2026
https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/220795
Backport c5b1e01ed897f417c370a225c6cdaaa14a1c142d
Requested by: @brad0
>From a48be23922e6a98849c8c081375e038c6892ae91 Mon Sep 17 00:00:00 2001
From: david04g <85910051+david04g at users.noreply.github.com>
Date: Wed, 2 Sep 2026 20:54:03 -0700
Subject: [PATCH] [MIPS] Fix GP-relative selection after select/add combine
(#215414)
## Summary
- Rewrite `$gp + select(...)` before MIPS instruction selection when a
select
arm contains a GP-relative relocation.
- Keep each `MipsISD::GPRel` attached to a selectable add operation.
- Add MIPS32 regression coverage and update the resulting MIPS64 checks.
## Background
The generic DAG combiner can transform:
select C, (add $gp, %gp_rel(A)), (add $gp, %gp_rel(B))
into:
add $gp, (select C, %gp_rel(A), %gp_rel(B))
This leaves `MipsISD::GPRel` as a standalone value, which MIPS
instruction
selection cannot match. The new MIPS DAG-to-DAG preprocessing step
restores
the distributive form before selection.
This follows the MIPS-local fixup direction discussed in #212163 and
avoids
introducing a target-independent hook.
## Testing
- Built `llc` on current `upstream/main`.
- Ran the exact MIPS32 static `-mgpopt -mattr=+noabicalls` reproducer.
- Ran:
- `llvm/test/CodeGen/Mips/cmov.ll`
- `llvm/test/CodeGen/Mips/llvm-ir/select-globaladdr.ll`
- Ran `check-llvm-codegen-mips`:
- 1040 discovered
- 1033 passed
- 3 unsupported
- 4 expected failures
- 0 unexpected failures
Fixes #212057
AI disclosure: I used OpenAI Codex to help with issue analysis, patch
development/review, and local validation. I personally reviewed and
understand the final code, tests, and PR description and take
responsibility
for the contribution.
(cherry picked from commit c5b1e01ed897f417c370a225c6cdaaa14a1c142d)
---
llvm/lib/Target/Mips/MipsISelDAGToDAG.cpp | 49 +++++++++++++++++++
llvm/lib/Target/Mips/MipsISelDAGToDAG.h | 2 +
llvm/test/CodeGen/Mips/cmov.ll | 6 +++
.../CodeGen/Mips/llvm-ir/select-globaladdr.ll | 7 ++-
4 files changed, 60 insertions(+), 4 deletions(-)
diff --git a/llvm/lib/Target/Mips/MipsISelDAGToDAG.cpp b/llvm/lib/Target/Mips/MipsISelDAGToDAG.cpp
index 36706243232c0..4e90d78048199 100644
--- a/llvm/lib/Target/Mips/MipsISelDAGToDAG.cpp
+++ b/llvm/lib/Target/Mips/MipsISelDAGToDAG.cpp
@@ -224,6 +224,55 @@ bool MipsDAGToDAGISel::selectVecAddAsVecSubIfProfitable(SDNode *Node) {
return true;
}
+void MipsDAGToDAGISel::PreprocessISelDAG() {
+ // The generic DAG combiner folds
+ //
+ // select C, (add $gp, %gp_rel(A)), (add $gp, %gp_rel(B))
+ //
+ // into
+ //
+ // add $gp, (select C, %gp_rel(A), %gp_rel(B)).
+ //
+ // A GP-relative relocation is only selectable as an operand of the add.
+ // Restore the original form before instruction selection so the relocation
+ // is never selected on its own.
+ bool MadeChange = false;
+ for (SDNode &Node : llvm::make_early_inc_range(CurDAG->allnodes())) {
+ if (Node.getOpcode() != ISD::ADD)
+ continue;
+
+ SDValue Base = Node.getOperand(0);
+ SDValue Sel = Node.getOperand(1);
+ if (Base.getOpcode() == ISD::SELECT)
+ std::swap(Base, Sel);
+
+ const auto *BaseReg = dyn_cast<RegisterSDNode>(Base);
+ if (!BaseReg ||
+ (BaseReg->getReg() != Mips::GP && BaseReg->getReg() != Mips::GP_64) ||
+ Sel.getOpcode() != ISD::SELECT)
+ continue;
+
+ SDValue TrueValue = Sel.getOperand(1);
+ SDValue FalseValue = Sel.getOperand(2);
+ if (TrueValue.getOpcode() != MipsISD::GPRel &&
+ FalseValue.getOpcode() != MipsISD::GPRel)
+ continue;
+
+ SDLoc DL(&Node);
+ EVT VT = Node.getValueType(0);
+ SDNodeFlags Flags = Node.getFlags();
+ TrueValue = CurDAG->getNode(ISD::ADD, DL, VT, Base, TrueValue, Flags);
+ FalseValue = CurDAG->getNode(ISD::ADD, DL, VT, Base, FalseValue, Flags);
+ SDValue NewSel =
+ CurDAG->getSelect(DL, VT, Sel.getOperand(0), TrueValue, FalseValue);
+ CurDAG->ReplaceAllUsesOfValueWith(SDValue(&Node, 0), NewSel);
+ MadeChange = true;
+ }
+
+ if (MadeChange)
+ CurDAG->RemoveDeadNodes();
+}
+
/// Select instructions not customized! Used for
/// expanded, promoted and normal instructions
void MipsDAGToDAGISel::Select(SDNode *Node) {
diff --git a/llvm/lib/Target/Mips/MipsISelDAGToDAG.h b/llvm/lib/Target/Mips/MipsISelDAGToDAG.h
index f19d72d94f806..1c90835526e43 100644
--- a/llvm/lib/Target/Mips/MipsISelDAGToDAG.h
+++ b/llvm/lib/Target/Mips/MipsISelDAGToDAG.h
@@ -124,6 +124,8 @@ class MipsDAGToDAGISel : public SelectionDAGISel {
/// add X, <-1, -1...> --> sub X, <1, 1...>
bool selectVecAddAsVecSubIfProfitable(SDNode *Node);
+ void PreprocessISelDAG() override;
+
void Select(SDNode *N) override;
virtual bool trySelect(SDNode *Node) = 0;
diff --git a/llvm/test/CodeGen/Mips/cmov.ll b/llvm/test/CodeGen/Mips/cmov.ll
index ee60b353b86b6..fba0526e7f5bc 100644
--- a/llvm/test/CodeGen/Mips/cmov.ll
+++ b/llvm/test/CodeGen/Mips/cmov.ll
@@ -5,6 +5,7 @@
; RUN: llc -mtriple=mips64el -mcpu=mips4 -relocation-model=pic < %s | FileCheck %s -check-prefixes=ALL,64-CMOV
; RUN: llc -mtriple=mips64el -mcpu=mips64 -relocation-model=pic < %s | FileCheck %s -check-prefixes=ALL,64-CMOV
; RUN: llc -mtriple=mips64el -mcpu=mips64r6 -relocation-model=pic < %s | FileCheck %s -check-prefixes=ALL,64-CMP
+; RUN: llc -mtriple=mips -mcpu=mips32 -relocation-model=static -mgpopt -mattr=+noabicalls < %s | FileCheck %s -check-prefix=GPREL
@i1 = global [3 x i32] [i32 1, i32 2, i32 3], align 4
@i3 = common global ptr null, align 4
@@ -51,6 +52,11 @@ entry:
; ALL-LABEL: cmov2:
+; GPREL-LABEL: cmov2:
+; GPREL-DAG: addiu $[[D:[0-9]+]], $gp, %gp_rel(d)
+; GPREL-DAG: addiu $[[C:[0-9]+]], $gp, %gp_rel(c)
+; GPREL: movn $[[D]], $[[C]], $4
+
; 32-CMOV-DAG: addiu $[[R1:[0-9]+]], ${{[0-9]+}}, %got(d)
; 32-CMOV-DAG: addiu $[[R0:[0-9]+]], ${{[0-9]+}}, %got(c)
; 32-CMOV-DAG: movn $[[R1]], $[[R0]], $4
diff --git a/llvm/test/CodeGen/Mips/llvm-ir/select-globaladdr.ll b/llvm/test/CodeGen/Mips/llvm-ir/select-globaladdr.ll
index 7e6a6b0dbcecd..a43525f4e2bbe 100644
--- a/llvm/test/CodeGen/Mips/llvm-ir/select-globaladdr.ll
+++ b/llvm/test/CodeGen/Mips/llvm-ir/select-globaladdr.ll
@@ -10,11 +10,10 @@ define ptr @tst_select_ptr_ptr(i1 %tobool.not) {
; MIPS64: # %bb.0: # %entry
; MIPS64-NEXT: sll $1, $4, 0
; MIPS64-NEXT: andi $1, $1, 1
-; MIPS64-NEXT: daddiu $2, $zero, %gp_rel(.str)
-; MIPS64-NEXT: daddiu $3, $zero, %gp_rel(.str.1)
-; MIPS64-NEXT: movn $2, $3, $1
+; MIPS64-NEXT: daddiu $2, $gp, %gp_rel(.str)
+; MIPS64-NEXT: daddiu $3, $gp, %gp_rel(.str.1)
; MIPS64-NEXT: jr $ra
-; MIPS64-NEXT: daddu $2, $gp, $2
+; MIPS64-NEXT: movn $2, $3, $1
entry:
%cond = select i1 %tobool.not, ptr @.str.1, ptr @.str
ret ptr %cond
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