Skip to content

[InstCombine] Do not combine shuffle+bitcast if the bitcast is eliminable. #135769

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

Merged
merged 4 commits into from
Apr 30, 2025
Merged

[InstCombine] Do not combine shuffle+bitcast if the bitcast is eliminable. #135769

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
23c096c
Precommit test.
rj-jesus Apr 14, 2025
9d9d585
[InstCombine] Do not combine shuffle+bitcast if the bitcast is elimin…
rj-jesus Apr 11, 2025
cbdcf1f
Add new test (transformation off).
rj-jesus Apr 29, 2025
06ecbae
Update tests (transformation on).
rj-jesus Apr 29, 2025
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
16 changes: 12 additions & 4 deletions llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -3029,10 +3029,18 @@ Instruction *InstCombinerImpl::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
SmallVector<BitCastInst *, 8> BCs;
DenseMap<Type *, Value *> NewBCs;
for (User *U : SVI.users())
if (BitCastInst *BC = dyn_cast<BitCastInst>(U))
if (!BC->use_empty())
// Only visit bitcasts that weren't previously handled.
BCs.push_back(BC);
if (BitCastInst *BC = dyn_cast<BitCastInst>(U)) {
// Only visit bitcasts that weren't previously handled.
if (BC->use_empty())
continue;
// Prefer to combine bitcasts of bitcasts before attempting this fold.
if (BC->hasOneUse()) {
auto *BC2 = dyn_cast<BitCastInst>(BC->user_back());
if (BC2 && isEliminableCastPair(BC, BC2))
continue;
}
BCs.push_back(BC);
}
for (BitCastInst *BC : BCs) {
unsigned BegIdx = Mask.front();
Type *TgtTy = BC->getDestTy();
Expand Down
35 changes: 35 additions & 0 deletions llvm/test/Transforms/InstCombine/shufflevec-bitcast.ll
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -235,3 +235,38 @@ define <3 x i4> @shuf_bitcast_wrong_size(<2 x i8> %v, i8 %x) {
%r = shufflevector <4 x i4> %b, <4 x i4> undef, <3 x i32> <i32 0, i32 1, i32 2>
ret <3 x i4> %r
}

; Negative test - chain of bitcasts.

define <16 x i8> @shuf_bitcast_chain(<8 x i32> %v) {
; CHECK-LABEL: @shuf_bitcast_chain(
; CHECK-NEXT: [[S:%.*]] = shufflevector <8 x i32> [[V:%.*]], <8 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
; CHECK-NEXT: [[C:%.*]] = bitcast <4 x i32> [[S]] to <16 x i8>
; CHECK-NEXT: ret <16 x i8> [[C]]
;
%s = shufflevector <8 x i32> %v, <8 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
%a = bitcast <4 x i32> %s to <2 x i64>
%b = bitcast <2 x i64> %a to i128
%c = bitcast i128 %b to <16 x i8>
ret <16 x i8> %c
}

; Same as above, but showing why it's not feasable to implement the reverse
; fold in VectorCombine (see #136998).

define <4 x i32> @shuf_bitcast_chain_2(<8 x i32> %v) {
; CHECK-LABEL: @shuf_bitcast_chain_2(
; CHECK-NEXT: [[S0:%.*]] = shufflevector <8 x i32> [[V:%.*]], <8 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
; CHECK-NEXT: [[S1:%.*]] = shufflevector <8 x i32> [[V]], <8 x i32> poison, <4 x i32> <i32 4, i32 5, i32 6, i32 7>
; CHECK-NEXT: [[R:%.*]] = or <4 x i32> [[S0]], [[S1]]
; CHECK-NEXT: ret <4 x i32> [[R]]
;
%s0 = shufflevector <8 x i32> %v, <8 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
%s1 = shufflevector <8 x i32> %v, <8 x i32> poison, <4 x i32> <i32 4, i32 5, i32 6, i32 7>
%b0 = bitcast <4 x i32> %s0 to i128
%b1 = bitcast <4 x i32> %s1 to i128
%c0 = bitcast i128 %b0 to <4 x i32>
%c1 = bitcast i128 %b1 to <4 x i32>
%r = or <4 x i32> %c0, %c1
ret <4 x i32> %r
}