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[InstCombine] Add limit for expansion of gep chains #147065

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Merged
merged 4 commits into from
Jul 23, 2025
Merged

[InstCombine] Add limit for expansion of gep chains #147065

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90ddc85
[InstCombine] Add limit for expansion of gep chains
nikic Jul 4, 2025
f2da011
Update for new one-use GEP expansion
nikic Jul 21, 2025
b8ca1c6
Adjust tests for new behavior
nikic Jul 21, 2025
5709fc1
Remove constant special case
nikic Jul 21, 2025
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22 changes: 21 additions & 1 deletion llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
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Expand Up @@ -2146,13 +2146,33 @@ CommonPointerBase CommonPointerBase::compute(Value *LHS, Value *RHS) {
return Base;
}

bool CommonPointerBase::isExpensive() const {
unsigned NumGEPs = 0;
auto ProcessGEPs = [&NumGEPs](ArrayRef<GEPOperator *> GEPs) {
bool SeenMultiUse = false;
for (GEPOperator *GEP : GEPs) {
// Only count multi-use GEPs, excluding the first one. For the first one,
// we will directly reuse the offset. For one-use GEPs, their offset will
// be folded into a multi-use GEP.
if (!GEP->hasOneUse()) {
if (SeenMultiUse)
++NumGEPs;
SeenMultiUse = true;
}
}
};
ProcessGEPs(LHSGEPs);
ProcessGEPs(RHSGEPs);
return NumGEPs > 2;
}

/// Optimize pointer differences into the same array into a size. Consider:
/// &A[10] - &A[0]: we should compile this to "10". LHS/RHS are the pointer
/// operands to the ptrtoint instructions for the LHS/RHS of the subtract.
Value *InstCombinerImpl::OptimizePointerDifference(Value *LHS, Value *RHS,
Type *Ty, bool IsNUW) {
CommonPointerBase Base = CommonPointerBase::compute(LHS, RHS);
if (!Base.Ptr)
if (!Base.Ptr || Base.isExpensive())
return nullptr;

// To avoid duplicating the offset arithmetic, rewrite the GEP to use the
Expand Down
2 changes: 1 addition & 1 deletion llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
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Expand Up @@ -712,7 +712,7 @@ Instruction *InstCombinerImpl::foldGEPICmp(GEPOperator *GEPLHS, Value *RHS,
};

CommonPointerBase Base = CommonPointerBase::compute(GEPLHS, RHS);
if (Base.Ptr == RHS && CanFold(Base.LHSNW)) {
if (Base.Ptr == RHS && CanFold(Base.LHSNW) && !Base.isExpensive()) {
// ((gep Ptr, OFFSET) cmp Ptr) ---> (OFFSET cmp 0).
Type *IdxTy = DL.getIndexType(GEPLHS->getType());
Value *Offset =
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3 changes: 3 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstCombineInternal.h
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Expand Up @@ -910,6 +910,9 @@ struct CommonPointerBase {
GEPNoWrapFlags RHSNW = GEPNoWrapFlags::all();

static CommonPointerBase compute(Value *LHS, Value *RHS);

/// Whether expanding the GEP chains is expensive.
bool isExpensive() const;
};

} // end namespace llvm
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132 changes: 132 additions & 0 deletions llvm/test/Transforms/InstCombine/icmp-gep.ll
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Expand Up @@ -849,3 +849,135 @@ define i1 @gep_mugtiple_ugt_inbounds_nusw(ptr %base, i64 %idx, i64 %idx2) {
%cmp = icmp ugt ptr %gep2, %base
ret i1 %cmp
}

define i1 @gep_multiple_multi_use_below_limit(ptr %base, i64 %idx1, i64 %idx2, i64 %idx3) {
; CHECK-LABEL: @gep_multiple_multi_use_below_limit(
; CHECK-NEXT: [[GEP3_IDX:%.*]] = shl i64 [[IDX3:%.*]], 2
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr i8, ptr [[GEP2:%.*]], i64 [[GEP3_IDX]]
; CHECK-NEXT: call void @use(ptr [[GEP3]])
; CHECK-NEXT: [[GEP2_IDX:%.*]] = shl i64 [[IDX2:%.*]], 2
; CHECK-NEXT: [[GEP4:%.*]] = getelementptr i8, ptr [[GEP3]], i64 [[GEP2_IDX]]
; CHECK-NEXT: call void @use(ptr [[GEP4]])
; CHECK-NEXT: [[GEP3_IDX1:%.*]] = shl i64 [[IDX4:%.*]], 2
; CHECK-NEXT: [[GEP5:%.*]] = getelementptr i8, ptr [[GEP4]], i64 [[GEP3_IDX1]]
; CHECK-NEXT: call void @use(ptr [[GEP5]])
; CHECK-NEXT: [[TMP1:%.*]] = add i64 [[GEP3_IDX]], [[GEP2_IDX]]
; CHECK-NEXT: [[TMP2:%.*]] = sub i64 0, [[GEP3_IDX1]]
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[TMP1]], [[TMP2]]
; CHECK-NEXT: ret i1 [[CMP]]
;
%gep1 = getelementptr i32, ptr %base, i64 %idx1
call void @use(ptr %gep1)
%gep2 = getelementptr i32, ptr %gep1, i64 %idx2
call void @use(ptr %gep2)
%gep3 = getelementptr i32, ptr %gep2, i64 %idx3
call void @use(ptr %gep3)
%cmp = icmp eq ptr %gep3, %base
ret i1 %cmp
}

define i1 @gep_multiple_multi_use_below_limit_extra_one_use_gep1(ptr %base, i64 %idx1, i64 %idx2, i64 %idx3, i64 %idx4) {
; CHECK-LABEL: @gep_multiple_multi_use_below_limit_extra_one_use_gep1(
; CHECK-NEXT: [[GEP1_IDX:%.*]] = shl i64 [[IDX1:%.*]], 2
; CHECK-NEXT: [[GEP1:%.*]] = getelementptr i8, ptr [[BASE:%.*]], i64 [[GEP1_IDX]]
; CHECK-NEXT: call void @use(ptr [[GEP1]])
; CHECK-NEXT: [[GEP2_IDX:%.*]] = shl i64 [[IDX2:%.*]], 2
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr i8, ptr [[GEP1]], i64 [[GEP2_IDX]]
; CHECK-NEXT: call void @use(ptr [[GEP2]])
; CHECK-NEXT: [[GEP3_IDX:%.*]] = shl i64 [[IDX3:%.*]], 2
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr i8, ptr [[GEP2]], i64 [[GEP3_IDX]]
; CHECK-NEXT: call void @use(ptr [[GEP3]])
; CHECK-NEXT: [[GEP4_IDX_NEG:%.*]] = mul i64 [[IDX4:%.*]], -4
; CHECK-NEXT: [[TMP1:%.*]] = add i64 [[GEP1_IDX]], [[GEP2_IDX]]
; CHECK-NEXT: [[TMP2:%.*]] = add i64 [[TMP1]], [[GEP3_IDX]]
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[TMP2]], [[GEP4_IDX_NEG]]
; CHECK-NEXT: ret i1 [[CMP]]
;
%gep1 = getelementptr i32, ptr %base, i64 %idx1
call void @use(ptr %gep1)
%gep2 = getelementptr i32, ptr %gep1, i64 %idx2
call void @use(ptr %gep2)
%gep3 = getelementptr i32, ptr %gep2, i64 %idx3
call void @use(ptr %gep3)
%gep4 = getelementptr i32, ptr %gep3, i64 %idx4
%cmp = icmp eq ptr %gep4, %base
ret i1 %cmp
}

define i1 @gep_multiple_multi_use_below_limit_extra_one_use_gep2(ptr %base, i64 %idx1, i64 %idx2, i64 %idx3, i64 %idx4) {
; CHECK-LABEL: @gep_multiple_multi_use_below_limit_extra_one_use_gep2(
; CHECK-NEXT: [[GEP1_IDX1:%.*]] = add i64 [[IDX1:%.*]], [[IDX2:%.*]]
; CHECK-NEXT: [[TMP1:%.*]] = shl i64 [[GEP1_IDX1]], 2
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr i8, ptr [[BASE:%.*]], i64 [[TMP1]]
; CHECK-NEXT: call void @use(ptr [[GEP2]])
; CHECK-NEXT: [[GEP3_IDX:%.*]] = shl i64 [[IDX3:%.*]], 2
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr i8, ptr [[GEP2]], i64 [[GEP3_IDX]]
; CHECK-NEXT: call void @use(ptr [[GEP3]])
; CHECK-NEXT: [[GEP4_IDX:%.*]] = shl i64 [[IDX4:%.*]], 2
; CHECK-NEXT: [[GEP4:%.*]] = getelementptr i8, ptr [[GEP3]], i64 [[GEP4_IDX]]
; CHECK-NEXT: call void @use(ptr [[GEP4]])
; CHECK-NEXT: [[TMP2:%.*]] = add i64 [[TMP1]], [[GEP3_IDX]]
; CHECK-NEXT: [[GEP4_IDX_NEG:%.*]] = sub i64 0, [[GEP4_IDX]]
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[TMP2]], [[GEP4_IDX_NEG]]
; CHECK-NEXT: ret i1 [[CMP]]
;
%gep1 = getelementptr i32, ptr %base, i64 %idx1
%gep2 = getelementptr i32, ptr %gep1, i64 %idx2
call void @use(ptr %gep2)
%gep3 = getelementptr i32, ptr %gep2, i64 %idx3
call void @use(ptr %gep3)
%gep4 = getelementptr i32, ptr %gep3, i64 %idx4
call void @use(ptr %gep4)
%cmp = icmp eq ptr %gep4, %base
ret i1 %cmp
}

define i1 @gep_multiple_multi_above_below_limit_consts(ptr %base, i64 %idx1, i64 %idx2) {
; CHECK-LABEL: @gep_multiple_multi_above_below_limit_consts(
; CHECK-NEXT: [[GEP1:%.*]] = getelementptr i8, ptr [[BASE:%.*]], i64 16
; CHECK-NEXT: call void @use(ptr [[GEP1]])
; CHECK-NEXT: [[GEP2:%.*]] = getelementptr i32, ptr [[GEP1]], i64 [[IDX1:%.*]]
; CHECK-NEXT: call void @use(ptr [[GEP2]])
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr i8, ptr [[GEP2]], i64 16
; CHECK-NEXT: call void @use(ptr [[GEP3]])
; CHECK-NEXT: [[GEP4:%.*]] = getelementptr i32, ptr [[GEP3]], i64 [[IDX2:%.*]]
; CHECK-NEXT: call void @use(ptr [[GEP4]])
; CHECK-NEXT: [[CMP:%.*]] = icmp eq ptr [[GEP4]], [[BASE]]
; CHECK-NEXT: ret i1 [[CMP]]
;
%gep1 = getelementptr i32, ptr %base, i64 4
call void @use(ptr %gep1)
%gep2 = getelementptr i32, ptr %gep1, i64 %idx1
call void @use(ptr %gep2)
%gep3 = getelementptr i32, ptr %gep2, i64 4
call void @use(ptr %gep3)
%gep4 = getelementptr i32, ptr %gep3, i64 %idx2
call void @use(ptr %gep4)
%cmp = icmp eq ptr %gep4, %base
ret i1 %cmp
}

define i1 @gep_multiple_multi_use_above_limit(ptr %base, i64 %idx1, i64 %idx2, i64 %idx3, i64 %idx4) {
; CHECK-LABEL: @gep_multiple_multi_use_above_limit(
; CHECK-NEXT: [[GEP4:%.*]] = getelementptr i32, ptr [[BASE:%.*]], i64 [[IDX1:%.*]]
; CHECK-NEXT: call void @use(ptr [[GEP4]])
; CHECK-NEXT: [[GEP3:%.*]] = getelementptr i32, ptr [[GEP4]], i64 [[IDX2:%.*]]
; CHECK-NEXT: call void @use(ptr [[GEP3]])
; CHECK-NEXT: [[GEP5:%.*]] = getelementptr i32, ptr [[GEP3]], i64 [[IDX3:%.*]]
; CHECK-NEXT: call void @use(ptr [[GEP5]])
; CHECK-NEXT: [[GEP6:%.*]] = getelementptr i32, ptr [[GEP5]], i64 [[IDX4:%.*]]
; CHECK-NEXT: call void @use(ptr [[GEP6]])
; CHECK-NEXT: [[CMP:%.*]] = icmp eq ptr [[GEP6]], [[BASE]]
; CHECK-NEXT: ret i1 [[CMP]]
;
%gep1 = getelementptr i32, ptr %base, i64 %idx1
call void @use(ptr %gep1)
%gep2 = getelementptr i32, ptr %gep1, i64 %idx2
call void @use(ptr %gep2)
%gep3 = getelementptr i32, ptr %gep2, i64 %idx3
call void @use(ptr %gep3)
%gep4 = getelementptr i32, ptr %gep3, i64 %idx4
call void @use(ptr %gep4)
%cmp = icmp eq ptr %gep4, %base
ret i1 %cmp
}
62 changes: 62 additions & 0 deletions llvm/test/Transforms/InstCombine/sub-gep.ll
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Expand Up @@ -1172,3 +1172,65 @@ define i64 @nuw_ptrdiff_mul_nsw_nneg_scale_multiuse(ptr %base, i64 %idx) {
%diff = sub nuw i64 %lhs, %rhs
ret i64 %diff
}

define i64 @multiple_geps_multi_use_below_limit(ptr %base, i64 %idx1, i64 %idx2, i64 %idx3, i64 %idx4) {
; CHECK-LABEL: @multiple_geps_multi_use_below_limit(
; CHECK-NEXT: [[P2:%.*]] = getelementptr inbounds nuw i8, ptr [[P1:%.*]], i64 [[IDX2:%.*]]
; CHECK-NEXT: call void @use(ptr [[P2]])
; CHECK-NEXT: [[P4:%.*]] = getelementptr inbounds nuw i8, ptr [[P2]], i64 [[IDX5:%.*]]
; CHECK-NEXT: call void @use(ptr [[P4]])
; CHECK-NEXT: [[P3:%.*]] = getelementptr inbounds nuw i8, ptr [[P1]], i64 [[IDX3:%.*]]
; CHECK-NEXT: call void @use(ptr [[P3]])
; CHECK-NEXT: [[P5:%.*]] = getelementptr inbounds nuw i8, ptr [[P3]], i64 [[IDX4:%.*]]
; CHECK-NEXT: call void @use(ptr [[P5]])
; CHECK-NEXT: [[TMP1:%.*]] = add nuw nsw i64 [[IDX2]], [[IDX5]]
; CHECK-NEXT: [[TMP2:%.*]] = add nuw nsw i64 [[IDX3]], [[IDX4]]
; CHECK-NEXT: [[GEPDIFF:%.*]] = sub nsw i64 [[TMP1]], [[TMP2]]
; CHECK-NEXT: ret i64 [[GEPDIFF]]
;
%p1 = getelementptr inbounds nuw i8, ptr %base, i64 %idx1
call void @use(ptr %p1)
%p2 = getelementptr inbounds nuw i8, ptr %p1, i64 %idx2
call void @use(ptr %p2)
%p3 = getelementptr inbounds nuw i8, ptr %base, i64 %idx3
call void @use(ptr %p3)
%p4 = getelementptr inbounds nuw i8, ptr %p3, i64 %idx4
call void @use(ptr %p4)
%i1 = ptrtoint ptr %p4 to i64
%i2 = ptrtoint ptr %p2 to i64
%d = sub i64 %i2, %i1
ret i64 %d
}

define i64 @multiple_geps_multi_use_above_limit(ptr %base, i64 %idx1, i64 %idx2, i64 %idx3, i64 %idx4, i64 %idx5) {
; CHECK-LABEL: @multiple_geps_multi_use_above_limit(
; CHECK-NEXT: [[P2:%.*]] = getelementptr inbounds nuw i8, ptr [[P1:%.*]], i64 [[IDX2:%.*]]
; CHECK-NEXT: call void @use(ptr [[P2]])
; CHECK-NEXT: [[P3:%.*]] = getelementptr inbounds nuw i8, ptr [[P2]], i64 [[IDX6:%.*]]
; CHECK-NEXT: call void @use(ptr [[P3]])
; CHECK-NEXT: [[P5:%.*]] = getelementptr inbounds nuw i8, ptr [[P1]], i64 [[TMP3:%.*]]
; CHECK-NEXT: call void @use(ptr [[P5]])
; CHECK-NEXT: [[P6:%.*]] = getelementptr inbounds nuw i8, ptr [[P5]], i64 [[IDX7:%.*]]
; CHECK-NEXT: call void @use(ptr [[P6]])
; CHECK-NEXT: [[P7:%.*]] = getelementptr inbounds nuw i8, ptr [[P6]], i64 [[IDX5:%.*]]
; CHECK-NEXT: call void @use(ptr [[P7]])
; CHECK-NEXT: [[I1:%.*]] = ptrtoint ptr [[P7]] to i64
; CHECK-NEXT: [[I2:%.*]] = ptrtoint ptr [[P3]] to i64
; CHECK-NEXT: [[D:%.*]] = sub i64 [[I2]], [[I1]]
; CHECK-NEXT: ret i64 [[D]]
;
%p1 = getelementptr inbounds nuw i8, ptr %base, i64 %idx1
call void @use(ptr %p1)
%p2 = getelementptr inbounds nuw i8, ptr %p1, i64 %idx2
call void @use(ptr %p2)
%p3 = getelementptr inbounds nuw i8, ptr %base, i64 %idx3
call void @use(ptr %p3)
%p4 = getelementptr inbounds nuw i8, ptr %p3, i64 %idx4
call void @use(ptr %p4)
%p5 = getelementptr inbounds nuw i8, ptr %p4, i64 %idx5
call void @use(ptr %p5)
%i1 = ptrtoint ptr %p5 to i64
%i2 = ptrtoint ptr %p2 to i64
%d = sub i64 %i2, %i1
ret i64 %d
}
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