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[InstCombine] Add test for freeze of GEP with recurrence offset (NFC) #145541

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Merged
merged 5 commits into from
Jun 26, 2025
Merged

[InstCombine] Add test for freeze of GEP with recurrence offset (NFC) #145541

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b32e341
[InstCombine] Add test for freeze of GEP with recurrence offset (NFC)
c-rhodes Jun 24, 2025
ded4097
update CHECK-LABEL
c-rhodes Jun 25, 2025
2ca19f1
address comments
c-rhodes Jun 25, 2025
a82060e
add test for GEP that can produce poison
c-rhodes Jun 25, 2025
b251c12
address comments
c-rhodes Jun 25, 2025
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98 changes: 98 additions & 0 deletions llvm/test/Transforms/InstCombine/freeze.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -877,6 +877,104 @@ exit: ; preds = %loop
ret void
}

; The recurrence for the GEP offset can't produce poison so the freeze should
; be pushed through to the ptr, but this is not currently supported.
define void @fold_phi_gep_phi_offset(ptr %init, ptr %end, i64 noundef %n) {
; CHECK-LABEL: @fold_phi_gep_phi_offset(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[I:%.*]] = phi ptr [ [[INIT:%.*]], [[ENTRY:%.*]] ], [ [[I_NEXT_FR:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[OFF:%.*]] = phi i64 [ [[N:%.*]], [[ENTRY]] ], [ [[OFF_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[OFF_NEXT]] = shl i64 [[OFF]], 3
; CHECK-NEXT: [[I_NEXT:%.*]] = getelementptr i8, ptr [[I]], i64 [[OFF_NEXT]]
; CHECK-NEXT: [[I_NEXT_FR]] = freeze ptr [[I_NEXT]]
; CHECK-NEXT: [[COND:%.*]] = icmp eq ptr [[I_NEXT_FR]], [[END:%.*]]
; CHECK-NEXT: br i1 [[COND]], label [[LOOP]], label [[EXIT:%.*]]
; CHECK: exit:
; CHECK-NEXT: ret void
;
entry:
br label %loop

loop: ; preds = %loop, %entry
%i = phi ptr [ %init, %entry ], [ %i.next.fr, %loop ]
%off = phi i64 [ %n, %entry ], [ %off.next, %loop ]
%off.next = shl i64 %off, 3
%i.next = getelementptr i8, ptr %i, i64 %off.next
%i.next.fr = freeze ptr %i.next
%cond = icmp eq ptr %i.next.fr, %end
br i1 %cond, label %loop, label %exit

exit: ; preds = %loop
ret void
}

; Offset is still guaranteed not to be poison, so the freeze could be moved
; here if we strip inbounds from the GEP, but this is not currently supported.
define void @fold_phi_gep_inbounds_phi_offset(ptr %init, ptr %end, i64 noundef %n) {
; CHECK-LABEL: @fold_phi_gep_inbounds_phi_offset(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[I:%.*]] = phi ptr [ [[INIT:%.*]], [[ENTRY:%.*]] ], [ [[I_NEXT_FR:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[OFF:%.*]] = phi i64 [ [[N:%.*]], [[ENTRY]] ], [ [[OFF_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[OFF_NEXT]] = shl i64 [[OFF]], 3
; CHECK-NEXT: [[I_NEXT:%.*]] = getelementptr inbounds i8, ptr [[I]], i64 [[OFF_NEXT]]
; CHECK-NEXT: [[I_NEXT_FR]] = freeze ptr [[I_NEXT]]
; CHECK-NEXT: [[COND:%.*]] = icmp eq ptr [[I_NEXT_FR]], [[END:%.*]]
; CHECK-NEXT: br i1 [[COND]], label [[LOOP]], label [[EXIT:%.*]]
; CHECK: exit:
; CHECK-NEXT: ret void
;
entry:
br label %loop

loop: ; preds = %loop, %entry
%i = phi ptr [ %init, %entry ], [ %i.next.fr, %loop ]
%off = phi i64 [ %n, %entry ], [ %off.next, %loop ]
%off.next = shl i64 %off, 3
%i.next = getelementptr inbounds i8, ptr %i, i64 %off.next
%i.next.fr = freeze ptr %i.next
%cond = icmp eq ptr %i.next.fr, %end
br i1 %cond, label %loop, label %exit

exit: ; preds = %loop
ret void
}

; GEP can produce poison, check freeze isn't moved.
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@david-arm david-arm Jun 25, 2025

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If I understand correctly the freeze could be moved here too if we strip inbounds from the gep and push the freeze up to the pointer %i? The offset %off.next is still guaranteed not to be poison or undef I think.

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@c-rhodes c-rhodes Jun 25, 2025

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If I understand correctly the freeze could be moved here too if we strip inbounds from the gep and push the freeze up to the pointer %i?

Yes I think so.

The offset %off.next is still guaranteed not to be poison or undef I think.

I don't believe so, %n needs to be marked noundef for that? I think this example is what you're referring to: https://godbolt.org/z/e8rGsz66W. Whereas Nikita wanted a test where the GEP can produce poison.

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@nikic nikic Jun 25, 2025

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Both variants probably have value. One requires just stripping poison, the other requires freezing an additional value.

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@c-rhodes c-rhodes Jun 25, 2025

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ok, I've also added a test for the inbounds GEP + noundef offset variant

define void @cant_fold_phi_gep_phi_offset(ptr %init, ptr %end, i64 %n) {
; CHECK-LABEL: @cant_fold_phi_gep_phi_offset(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[I:%.*]] = phi ptr [ [[INIT:%.*]], [[ENTRY:%.*]] ], [ [[I_NEXT_FR:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[OFF:%.*]] = phi i64 [ [[N:%.*]], [[ENTRY]] ], [ [[OFF_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[OFF_NEXT]] = shl i64 [[OFF]], 3
; CHECK-NEXT: [[I_NEXT:%.*]] = getelementptr inbounds i8, ptr [[I]], i64 [[OFF_NEXT]]
; CHECK-NEXT: [[I_NEXT_FR]] = freeze ptr [[I_NEXT]]
; CHECK-NEXT: [[COND:%.*]] = icmp eq ptr [[I_NEXT_FR]], [[END:%.*]]
; CHECK-NEXT: br i1 [[COND]], label [[LOOP]], label [[EXIT:%.*]]
; CHECK: exit:
; CHECK-NEXT: ret void
;
entry:
br label %loop

loop: ; preds = %loop, %entry
%i = phi ptr [ %init, %entry ], [ %i.next.fr, %loop ]
%off = phi i64 [ %n, %entry ], [ %off.next, %loop ]
%off.next = shl i64 %off, 3
%i.next = getelementptr inbounds i8, ptr %i, i64 %off.next
%i.next.fr = freeze ptr %i.next
%cond = icmp eq ptr %i.next.fr, %end
br i1 %cond, label %loop, label %exit

exit: ; preds = %loop
ret void
}

define void @fold_phi_multiple_insts(i32 %init, i32 %n) {
; CHECK-LABEL: @fold_phi_multiple_insts(
; CHECK-NEXT: entry:
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