[Mips] Fix cttz.i32 fails to lower on mips16 #179633
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MIPS16 cannot handle constant pools created by CTTZ table lookup expansion. This causes "Cannot select" errors when trying to select MipsISD::Lo nodes for constant pool addresses. The fix adds a virtual method `canUseConstantPoolForCTTZ()` to TargetLowering. MIPS16 overrides this to return false, using ISD::CTPOP. Fix llvm#61055.
llvmbot
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backend:MIPS
llvm:globalisel
llvm:SelectionDAG
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@llvm/pr-subscribers-llvm-selectiondag @llvm/pr-subscribers-backend-mips Author: None (yingopq) ChangesMIPS16 cannot handle constant pools created by CTTZ table lookup expansion. This causes "Cannot select" errors when trying to select MipsISD::Lo nodes for constant pool addresses. The fix adds a virtual method Fix #61055. Full diff: https://github.com/llvm/llvm-project/pull/179633.diff 5 Files Affected:
diff --git a/llvm/include/llvm/CodeGen/TargetLowering.h b/llvm/include/llvm/CodeGen/TargetLowering.h
index ada4ffd3bcc89..27954affe7b61 100644
--- a/llvm/include/llvm/CodeGen/TargetLowering.h
+++ b/llvm/include/llvm/CodeGen/TargetLowering.h
@@ -727,6 +727,9 @@ class LLVM_ABI TargetLoweringBase {
return false;
}
+ /// Return true if constant pool can be used for CTTZ expansion.
+ virtual bool canUseConstantPoolForCTTZ() const { return true; }
+
/// Return true if ctlz instruction is fast.
virtual bool isCtlzFast() const {
return false;
diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index a10b93063dc27..2c02e761b1b03 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -9691,8 +9691,9 @@ SDValue TargetLowering::expandCTTZ(SDNode *Node, SelectionDAG &DAG) const {
// Emit Table Lookup if ISD::CTPOP used in the fallback path below is going
// to be expanded or converted to a libcall.
+ const TargetLowering &TLI = DAG.getTargetLoweringInfo();
if (!VT.isVector() && !isOperationLegalOrCustomOrPromote(ISD::CTPOP, VT) &&
- !isOperationLegal(ISD::CTLZ, VT))
+ !isOperationLegal(ISD::CTLZ, VT) && TLI.canUseConstantPoolForCTTZ())
if (SDValue V = CTTZTableLookup(Node, DAG, dl, VT, Op, NumBitsPerElt))
return V;
diff --git a/llvm/lib/Target/Mips/MipsISelLowering.cpp b/llvm/lib/Target/Mips/MipsISelLowering.cpp
index f7e2825edcc5d..d92b70633fb94 100644
--- a/llvm/lib/Target/Mips/MipsISelLowering.cpp
+++ b/llvm/lib/Target/Mips/MipsISelLowering.cpp
@@ -1190,6 +1190,11 @@ bool MipsTargetLowering::isCheapToSpeculateCtlz(Type *Ty) const {
return Subtarget.hasMips32();
}
+bool MipsTargetLowering::canUseConstantPoolForCTTZ() const {
+ // MIPS16 cannot use constant pools for CTTZ expansion.
+ return !Subtarget.inMips16Mode();
+}
+
bool MipsTargetLowering::hasBitTest(SDValue X, SDValue Y) const {
// We can use ANDI+SLTIU as a bit test. Y contains the bit position.
// For MIPSR2 or later, we may be able to use the `ext` instruction or its
diff --git a/llvm/lib/Target/Mips/MipsISelLowering.h b/llvm/lib/Target/Mips/MipsISelLowering.h
index 806fb59f5ff3e..17fd4db0ba152 100644
--- a/llvm/lib/Target/Mips/MipsISelLowering.h
+++ b/llvm/lib/Target/Mips/MipsISelLowering.h
@@ -81,6 +81,7 @@ class TargetRegisterClass;
bool isCheapToSpeculateCttz(Type *Ty) const override;
bool isCheapToSpeculateCtlz(Type *Ty) const override;
+ bool canUseConstantPoolForCTTZ() const override;
bool hasBitTest(SDValue X, SDValue Y) const override;
bool shouldFoldConstantShiftPairToMask(const SDNode *N) const override;
diff --git a/llvm/test/CodeGen/Mips/GlobalISel/llvm-ir/cttz-mips16.ll b/llvm/test/CodeGen/Mips/GlobalISel/llvm-ir/cttz-mips16.ll
new file mode 100644
index 0000000000000..7dbb4fc567a8b
--- /dev/null
+++ b/llvm/test/CodeGen/Mips/GlobalISel/llvm-ir/cttz-mips16.ll
@@ -0,0 +1,61 @@
+; RUN: llc -mtriple=mipsel-linux-gnu -mcpu=mips32 -mattr=+mips16 -verify-machineinstrs < %s | FileCheck %s -check-prefixes=MIPS16
+
+define i32 @cttz_i32(i32 %a) {
+; MIPS16-LABEL: cttz_i32:
+; MIPS16: # %bb.0: # %entry
+; MIPS16-NEXT: not $2, $4
+; MIPS16-NEXT: addiu $4, -1 # 16 bit inst
+; MIPS16-NEXT: and $4, $2
+; MIPS16-NEXT: srl $2, $4, 1
+; MIPS16-NEXT: lw $3, $CPI0_0 # 16 bit inst
+; MIPS16-NEXT: and $3, $2
+; MIPS16-NEXT: subu $2, $4, $3
+; MIPS16-NEXT: lw $3, $CPI0_1 # 16 bit inst
+; MIPS16-NEXT: srl $4, $2, 2
+; MIPS16-NEXT: and $2, $3
+; MIPS16-NEXT: and $4, $3
+; MIPS16-NEXT: addu $2, $2, $4
+; MIPS16-NEXT: srl $3, $2, 4
+; MIPS16-NEXT: addu $2, $2, $3
+; MIPS16-NEXT: lw $3, $CPI0_2 # 16 bit inst
+; MIPS16-NEXT: and $3, $2
+; MIPS16-NEXT: lw $2, $CPI0_3 # 16 bit inst
+; MIPS16-NEXT: mult $3, $2
+; MIPS16-NEXT: mflo $2
+; MIPS16-NEXT: srl $2, $2, 24
+; MIPS16-NEXT: jrc $ra
+
+entry:
+ %0 = call i32 @llvm.cttz.i32(i32 %a, i1 false)
+ ret i32 %0
+}
+declare i32 @llvm.cttz.i32(i32, i1 immarg)
+
+define i64 @cttz_i64(i64 %a) {
+; MIPS16-LABEL: cttz_i64:
+; MIPS16: # %bb.1: # %entry
+; MIPS16-NEXT: not $4, $5
+; MIPS16-NEXT: addiu $5, -1 # 16 bit inst
+; MIPS16-NEXT: and $5, $4
+; MIPS16-NEXT: srl $4, $5, 1
+; MIPS16-NEXT: and $4, $7
+; MIPS16-NEXT: subu $4, $5, $4
+; MIPS16-NEXT: srl $5, $4, 2
+; MIPS16-NEXT: and $4, $6
+; MIPS16-NEXT: and $5, $6
+; MIPS16-NEXT: addu $4, $4, $5
+; MIPS16-NEXT: srl $5, $4, 4
+; MIPS16-NEXT: addu $4, $4, $5
+; MIPS16-NEXT: and $4, $3
+; MIPS16-NEXT: mult $4, $2
+; MIPS16-NEXT: mflo $2
+; MIPS16-NEXT: srl $2, $2, 24
+; MIPS16-NEXT: addiu $2, 32 # 16 bit inst
+; MIPS16-NEXT: li $3, 0
+; MIPS16-NEXT: jrc $ra
+
+entry:
+ %0 = call i64 @llvm.cttz.i64(i64 %a, i1 false)
+ ret i64 %0
+}
+declare i64 @llvm.cttz.i64(i64, i1 immarg)
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RKSimon
reviewed
Feb 4, 2026
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RKSimon
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RKSimon
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Are we not just missing a legality check inside CTTZTableLookup? I'd really like to avoid yet another TLI hack.
arsenm
requested changes
Feb 4, 2026
Comment on lines
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| /// Return true if constant pool can be used for CTTZ expansion. | ||
| virtual bool canUseConstantPoolForCTTZ() const { return true; } | ||
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This doesn't make any sense and is too specific
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MIPS16 cannot handle constant pools created by CTTZ table lookup expansion. This causes "Cannot select" errors when trying to select MipsISD::Lo nodes for constant pool addresses.
The fix adds a virtual method
canUseConstantPoolForCTTZ()to TargetLowering. MIPS16 overrides this to return false, using ISD::CTPOP.Fix #61055.