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[LV] Use SCEV to check if minimum iteration check is known. #111310

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Oct 18, 2024
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[LV] Use SCEV to check if minimum iteration check is known. #111310

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512aa0d
[LV] Use SCEV to check if minimum iteration check is known.
fhahn Oct 6, 2024
a2e66a5
Merge remote-tracking branch 'origin/main' into lv-use-scev-for-min-iter
fhahn Oct 9, 2024
86ea8c3
!fixup retrieve trip count SCEV from Count value directly.
fhahn Oct 9, 2024
41e31e9
Merge remote-tracking branch 'origin/main' into lv-use-scev-for-min-iter
fhahn Oct 9, 2024
8a89ff3
!fixup address latest comments, thanks
fhahn Oct 9, 2024
0030b1b
Merge remote-tracking branch 'origin/main' into lv-use-scev-for-min-iter
fhahn Oct 10, 2024
5339f4d
!fixup address latest comments, thanks!
fhahn Oct 10, 2024
31cee3e
Merge remote-tracking branch 'origin/main' into lv-use-scev-for-min-iter
fhahn Oct 13, 2024
cacf91b
!fixup addres comments, thanks!
fhahn Oct 13, 2024
aac81c6
Merge remote-tracking branch 'origin/main' into lv-use-scev-for-min-iter
fhahn Oct 16, 2024
5e455de
!fixup wrap code only used in assertion in NDEBUG
fhahn Oct 16, 2024
092c2d6
Merge remote-tracking branch 'origin/main' into lv-use-scev-for-min-iter
fhahn Oct 18, 2024
660fe44
!fixup address latest comments, thanks!
fhahn Oct 18, 2024
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39 changes: 32 additions & 7 deletions llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
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Expand Up @@ -2440,12 +2440,28 @@ void InnerLoopVectorizer::emitIterationCountCheck(BasicBlock *Bypass) {
};

TailFoldingStyle Style = Cost->getTailFoldingStyle();
if (Style == TailFoldingStyle::None)
CheckMinIters =
Builder.CreateICmp(P, Count, CreateStep(), "min.iters.check");
else if (VF.isScalable() &&
!isIndvarOverflowCheckKnownFalse(Cost, VF, UF) &&
Style != TailFoldingStyle::DataAndControlFlowWithoutRuntimeCheck) {
if (Style == TailFoldingStyle::None) {
Value *Step = CreateStep();
ScalarEvolution &SE = *PSE.getSE();
// TODO: Emit unconditional branch to vector preheader instead of
// conditional branch with known condition.
const SCEV *TripCountSCEV = SE.getSCEV(Count);
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nit: can also apply loop guards to TripCountSCEV here.

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Done, thanks!

// Check if the trip count is < the step.
if (SE.isKnownPredicate(P, SE.applyLoopGuards(TripCountSCEV, OrigLoop),
SE.getSCEV(Step))) {
// TODO: Should not attempt to vectorize when the vector loop is known to
// never execute.
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Suggested change
// TODO: Should not attempt to vectorize when the vector loop is known to
// never execute.
// TODO: Ensure step is at most the trip count when determining max VF and UF, w/o tail folding.

?

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Updated, thanks!

CheckMinIters = Builder.getTrue();
} else if (!SE.isKnownPredicate(CmpInst::getInversePredicate(P),
SE.applyLoopGuards(TripCountSCEV, OrigLoop),
SE.getSCEV(Step))) {
// Only generate the minimum iteration check only if we cannot prove the
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Suggested change
// Only generate the minimum iteration check only if we cannot prove the
// Generate the minimum iteration check only if we cannot prove the

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Updated, thanks!

// check is known to be false.
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Suggested change
// check is known to be false.
// check is known to be true, or known to be false.

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Updated, thanks!

CheckMinIters = Builder.CreateICmp(P, Count, Step, "min.iters.check");
}
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Suggested change
}
}
// else step is known to be smaller than trip count, use CheckMinIters preset to false.

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added, thanks!

} else if (VF.isScalable() &&
!isIndvarOverflowCheckKnownFalse(Cost, VF, UF) &&
Style != TailFoldingStyle::DataAndControlFlowWithoutRuntimeCheck) {
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The optimization you're doing here applies to the check in this if-block as well. Maybe factor out an getOptimizedCompare lambda or something?

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The runtime comparison introduced below checks for overflow, in case the overflow check is not known (to be false) at compile time. Perhaps worth asserting that this predicate is indeed unknown to SCEV.

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Added an assert, thanks!

// vscale is not necessarily a power-of-2, which means we cannot guarantee
// an overflow to zero when updating induction variables and so an
// additional overflow check is required before entering the vector loop.
Expand All @@ -2455,8 +2471,17 @@ void InnerLoopVectorizer::emitIterationCountCheck(BasicBlock *Bypass) {
ConstantInt::get(CountTy, cast<IntegerType>(CountTy)->getMask());
Value *LHS = Builder.CreateSub(MaxUIntTripCount, Count);

Value *Step = CreateStep();
ScalarEvolution &SE = *PSE.getSE();
// Check if we can prove that the trip count is >= the step.
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The condition below checks if the trip count is too close to UMax - such that bumping it by step overflows, rather than checking if trip count can be proven to be >= step. Another TODO?

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TODO to clarify in the naming?

// TODO: Emit unconditional branch to vector preheader instead of
// conditional branch with known condition.
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No known condition is used below.

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Dropped, thanks!

const SCEV *TripCountSCEV = SE.getSCEV(LHS);
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nit: can also apply loop guards to TripCountSCEV here.

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Done, thanks!

assert(!SE.isKnownPredicate(CmpInst::getInversePredicate(ICmpInst::ICMP_ULT),
SE.applyLoopGuards(TripCountSCEV, OrigLoop),
SE.getSCEV(Step)) && "SCEV unexpectedly proved overflow check to be known);
// Don't execute the vector loop if (UMax - n) < (VF * UF).
CheckMinIters = Builder.CreateICmp(ICmpInst::ICMP_ULT, LHS, CreateStep());
CheckMinIters = Builder.CreateICmp(ICmpInst::ICMP_ULT, LHS, Step);
}
// Create new preheader for vector loop.
Expand Down
3 changes: 1 addition & 2 deletions llvm/test/Transforms/LoopVectorize/AArch64/eliminate-tail-predication.ll
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Expand Up @@ -11,8 +11,7 @@ define void @f1(ptr %A) #0 {
; CHECK-NEXT: entry:
; CHECK-NEXT: [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
; CHECK-NEXT: [[TMP1:%.*]] = mul i64 [[TMP0]], 4
; CHECK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 1024, [[TMP1]]
; CHECK-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; CHECK-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
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Step of 4 * vscale is known to be smaller than count of 1024, based on vscale_range(1,16) attribute?

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Yep

; CHECK: vector.ph:
; CHECK-NEXT: [[TMP2:%.*]] = call i64 @llvm.vscale.i64()
; CHECK-NEXT: [[TMP3:%.*]] = mul i64 [[TMP2]], 4
Expand Down
44 changes: 9 additions & 35 deletions llvm/test/Transforms/LoopVectorize/AArch64/masked-call.ll
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Expand Up @@ -11,10 +11,7 @@ target triple = "aarch64-unknown-linux-gnu"
define void @test_widen(ptr noalias %a, ptr readnone %b) #4 {
; TFNONE-LABEL: @test_widen(
; TFNONE-NEXT: entry:
; TFNONE-NEXT: [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
; TFNONE-NEXT: [[TMP1:%.*]] = mul i64 [[TMP0]], 2
; TFNONE-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 1025, [[TMP1]]
; TFNONE-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; TFNONE-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; TFNONE: vector.ph:
; TFNONE-NEXT: [[TMP2:%.*]] = call i64 @llvm.vscale.i64()
; TFNONE-NEXT: [[TMP3:%.*]] = mul i64 [[TMP2]], 2
Expand Down Expand Up @@ -146,10 +143,7 @@ for.cond.cleanup:
define void @test_if_then(ptr noalias %a, ptr readnone %b) #4 {
; TFNONE-LABEL: @test_if_then(
; TFNONE-NEXT: entry:
; TFNONE-NEXT: [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
; TFNONE-NEXT: [[TMP1:%.*]] = mul i64 [[TMP0]], 2
; TFNONE-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 1025, [[TMP1]]
; TFNONE-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; TFNONE-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; TFNONE: vector.ph:
; TFNONE-NEXT: [[TMP2:%.*]] = call i64 @llvm.vscale.i64()
; TFNONE-NEXT: [[TMP3:%.*]] = mul i64 [[TMP2]], 2
Expand Down Expand Up @@ -310,10 +304,7 @@ for.cond.cleanup:
define void @test_widen_if_then_else(ptr noalias %a, ptr readnone %b) #4 {
; TFNONE-LABEL: @test_widen_if_then_else(
; TFNONE-NEXT: entry:
; TFNONE-NEXT: [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
; TFNONE-NEXT: [[TMP1:%.*]] = mul i64 [[TMP0]], 2
; TFNONE-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 1025, [[TMP1]]
; TFNONE-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; TFNONE-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; TFNONE: vector.ph:
; TFNONE-NEXT: [[TMP2:%.*]] = call i64 @llvm.vscale.i64()
; TFNONE-NEXT: [[TMP3:%.*]] = mul i64 [[TMP2]], 2
Expand Down Expand Up @@ -490,10 +481,7 @@ for.cond.cleanup:
define void @test_widen_nomask(ptr noalias %a, ptr readnone %b) #4 {
; TFNONE-LABEL: @test_widen_nomask(
; TFNONE-NEXT: entry:
; TFNONE-NEXT: [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
; TFNONE-NEXT: [[TMP1:%.*]] = mul i64 [[TMP0]], 2
; TFNONE-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 1025, [[TMP1]]
; TFNONE-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; TFNONE-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; TFNONE: vector.ph:
; TFNONE-NEXT: [[TMP2:%.*]] = call i64 @llvm.vscale.i64()
; TFNONE-NEXT: [[TMP3:%.*]] = mul i64 [[TMP2]], 2
Expand Down Expand Up @@ -548,11 +536,6 @@ define void @test_widen_nomask(ptr noalias %a, ptr readnone %b) #4 {
;
; TFFALLBACK-LABEL: @test_widen_nomask(
; TFFALLBACK-NEXT: entry:
; TFFALLBACK-NEXT: [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
; TFFALLBACK-NEXT: [[TMP1:%.*]] = mul i64 [[TMP0]], 2
; TFFALLBACK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 1025, [[TMP1]]
; TFFALLBACK-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; TFFALLBACK: vector.ph:
; TFFALLBACK-NEXT: [[TMP2:%.*]] = call i64 @llvm.vscale.i64()
; TFFALLBACK-NEXT: [[TMP3:%.*]] = mul i64 [[TMP2]], 2
; TFFALLBACK-NEXT: [[N_MOD_VF:%.*]] = urem i64 1025, [[TMP3]]
Expand All @@ -561,20 +544,17 @@ define void @test_widen_nomask(ptr noalias %a, ptr readnone %b) #4 {
; TFFALLBACK-NEXT: [[TMP5:%.*]] = mul i64 [[TMP4]], 2
; TFFALLBACK-NEXT: br label [[VECTOR_BODY:%.*]]
; TFFALLBACK: vector.body:
; TFFALLBACK-NEXT: [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
; TFFALLBACK-NEXT: [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH:%.*]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
; TFFALLBACK-NEXT: [[TMP6:%.*]] = getelementptr i64, ptr [[B:%.*]], i64 [[INDEX]]
; TFFALLBACK-NEXT: [[WIDE_LOAD:%.*]] = load <vscale x 2 x i64>, ptr [[TMP6]], align 8
; TFFALLBACK-NEXT: [[TMP7:%.*]] = call <vscale x 2 x i64> @foo_vector_nomask(<vscale x 2 x i64> [[WIDE_LOAD]])
; TFFALLBACK-NEXT: [[TMP8:%.*]] = getelementptr inbounds i64, ptr [[A:%.*]], i64 [[INDEX]]
; TFFALLBACK-NEXT: store <vscale x 2 x i64> [[TMP7]], ptr [[TMP8]], align 8
; TFFALLBACK-NEXT: [[INDEX_NEXT]] = add nuw i64 [[INDEX]], [[TMP5]]
; TFFALLBACK-NEXT: [[TMP9:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
; TFFALLBACK-NEXT: br i1 [[TMP9]], label [[SCALAR_PH]], label [[VECTOR_BODY]], !llvm.loop [[LOOP5:![0-9]+]]
; TFFALLBACK: scalar.ph:
; TFFALLBACK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[N_VEC]], [[VECTOR_BODY]] ]
; TFFALLBACK-NEXT: br label [[FOR_BODY:%.*]]
; TFFALLBACK-NEXT: br i1 [[TMP9]], label [[FOR_BODY:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP5:![0-9]+]]
; TFFALLBACK: for.body:
; TFFALLBACK-NEXT: [[INDVARS_IV:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[INDVARS_IV_NEXT:%.*]], [[FOR_BODY]] ]
; TFFALLBACK-NEXT: [[INDVARS_IV:%.*]] = phi i64 [ [[INDVARS_IV_NEXT:%.*]], [[FOR_BODY]] ], [ [[N_VEC]], [[VECTOR_BODY]] ]
; TFFALLBACK-NEXT: [[GEP:%.*]] = getelementptr i64, ptr [[B]], i64 [[INDVARS_IV]]
; TFFALLBACK-NEXT: [[LOAD:%.*]] = load i64, ptr [[GEP]], align 8
; TFFALLBACK-NEXT: [[CALL:%.*]] = call i64 @foo(i64 [[LOAD]]) #[[ATTR5:[0-9]+]]
Expand Down Expand Up @@ -626,10 +606,7 @@ for.cond.cleanup:
define void @test_widen_optmask(ptr noalias %a, ptr readnone %b) #4 {
; TFNONE-LABEL: @test_widen_optmask(
; TFNONE-NEXT: entry:
; TFNONE-NEXT: [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
; TFNONE-NEXT: [[TMP1:%.*]] = mul i64 [[TMP0]], 2
; TFNONE-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 1025, [[TMP1]]
; TFNONE-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; TFNONE-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; TFNONE: vector.ph:
; TFNONE-NEXT: [[TMP2:%.*]] = call i64 @llvm.vscale.i64()
; TFNONE-NEXT: [[TMP3:%.*]] = mul i64 [[TMP2]], 2
Expand Down Expand Up @@ -791,10 +768,7 @@ for.cond.cleanup:
define double @test_widen_fmuladd_and_call(ptr noalias %a, ptr readnone %b, double %m) #4 {
; TFNONE-LABEL: @test_widen_fmuladd_and_call(
; TFNONE-NEXT: entry:
; TFNONE-NEXT: [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
; TFNONE-NEXT: [[TMP1:%.*]] = mul i64 [[TMP0]], 2
; TFNONE-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 1025, [[TMP1]]
; TFNONE-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; TFNONE-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
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ditto with vscale_range(2,16)

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Yep

; TFNONE: vector.ph:
; TFNONE-NEXT: [[TMP2:%.*]] = call i64 @llvm.vscale.i64()
; TFNONE-NEXT: [[TMP3:%.*]] = mul i64 [[TMP2]], 2
Expand Down
3 changes: 1 addition & 2 deletions llvm/test/Transforms/LoopVectorize/AArch64/pr60831-sve-inv-store-crash.ll
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Expand Up @@ -10,8 +10,7 @@ define void @test_invar_gep(ptr %dst) #0 {
; CHECK-NEXT: entry:
; CHECK-NEXT: [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
; CHECK-NEXT: [[TMP1:%.*]] = mul i64 [[TMP0]], 4
; CHECK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 100, [[TMP1]]
; CHECK-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; CHECK-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; CHECK: vector.ph:
; CHECK-NEXT: [[TMP2:%.*]] = call i64 @llvm.vscale.i64()
; CHECK-NEXT: [[TMP3:%.*]] = mul i64 [[TMP2]], 4
Expand Down
3 changes: 1 addition & 2 deletions llvm/test/Transforms/LoopVectorize/AArch64/sve-tail-folding.ll
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Expand Up @@ -757,8 +757,7 @@ define void @simple_memset_trip1024(i32 %val, ptr %ptr, i64 %n) #0 {
; CHECK-NEXT: entry:
; CHECK-NEXT: [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
; CHECK-NEXT: [[TMP1:%.*]] = mul i64 [[TMP0]], 4
; CHECK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 1024, [[TMP1]]
; CHECK-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; CHECK-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; CHECK: vector.ph:
; CHECK-NEXT: [[TMP2:%.*]] = call i64 @llvm.vscale.i64()
; CHECK-NEXT: [[TMP3:%.*]] = mul i64 [[TMP2]], 4
Expand Down
5 changes: 1 addition & 4 deletions llvm/test/Transforms/LoopVectorize/AArch64/wider-VF-for-callinst.ll
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Expand Up @@ -7,10 +7,7 @@ target triple = "aarch64-unknown-linux-gnu"
define void @test_widen(ptr noalias %a, ptr readnone %b) #1 {
; WIDE-LABEL: @test_widen(
; WIDE-NEXT: entry:
; WIDE-NEXT: [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
; WIDE-NEXT: [[TMP1:%.*]] = mul i64 [[TMP0]], 4
; WIDE-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 1025, [[TMP1]]
; WIDE-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; WIDE-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
; WIDE: vector.ph:
; WIDE-NEXT: [[TMP2:%.*]] = call i64 @llvm.vscale.i64()
; WIDE-NEXT: [[TMP3:%.*]] = mul i64 [[TMP2]], 4
Expand Down
3 changes: 1 addition & 2 deletions llvm/test/Transforms/LoopVectorize/if-reduction.ll
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Expand Up @@ -1668,8 +1668,7 @@ define i32 @fcmp_0_sub_select1(ptr noalias %x, i32 %N) nounwind readonly {
; CHECK: [[FOR_HEADER]]:
; CHECK-NEXT: [[ZEXT:%.*]] = zext i32 [[N]] to i64
; CHECK-NEXT: [[TMP0:%.*]] = sub i64 0, [[ZEXT]]
; CHECK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 [[TMP0]], 4
; CHECK-NEXT: br i1 [[MIN_ITERS_CHECK]], label %[[SCALAR_PH:.*]], label %[[VECTOR_PH:.*]]
; CHECK-NEXT: br i1 false, label %[[SCALAR_PH:.*]], label %[[VECTOR_PH:.*]]
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Somewhat confusing (min) iter check here, bumping %indvars.iv.next = sub nuw nsw i64 %indvars.iv, 1 repeatedly starting with %indvars.iv set to zero?

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Agreed, might be worth fixing independently.

The simplification is fine for the input I think, BTC is (-1 + (-1 * (zext i32 %N to i64))<nsw>)<nsw>, trip count with info from the dominating loop guard is (-1 * (zext i32 (1 smax %N) to i64))<nsw> which should be u>= 4. https://llvm.godbolt.org/z/1EMWbGb81

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Sure, worth fixing test independently, before or after. Subtracting 1 from 0 on first iteration, and implicitly casting the above negative BTC and trip count to unsigned, defy the claimed nuw.

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Worth leaving behind a FIXME note.

; CHECK: [[VECTOR_PH]]:
; CHECK-NEXT: [[N_MOD_VF:%.*]] = urem i64 [[TMP0]], 4
; CHECK-NEXT: [[N_VEC:%.*]] = sub i64 [[TMP0]], [[N_MOD_VF]]
Expand Down
3 changes: 1 addition & 2 deletions llvm/test/Transforms/LoopVectorize/version-stride-with-integer-casts.ll
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Expand Up @@ -423,8 +423,7 @@ define void @zext_of_i1_stride(i1 %g, ptr %dst) mustprogress {
; CHECK-NEXT: [[G_64:%.*]] = zext i1 [[G]] to i64
; CHECK-NEXT: [[TMP0:%.*]] = udiv i64 15, [[G_64]]
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Better divide 15 by G_64 after scevcheck'ing below that G is 1 (not 0), than before?

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Yep would probably be better for this particular check. There are other SCEV checks that are much more expensive (like wrapping checks), so we would probably need to distinguish between them.

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Sure, guards should be ordered according to cost and frequency, but in this case a potential division by zero is introduced, unguarded.

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Worth leaving behind a FIXME note.

; CHECK-NEXT: [[TMP1:%.*]] = add nuw nsw i64 [[TMP0]], 1
; CHECK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 [[TMP1]], 4
; CHECK-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_SCEVCHECK:%.*]]
; CHECK-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_SCEVCHECK:%.*]]
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Count of 16 (assuming G = 1) is known to be greater than step of 4.

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Yes, the step of 4 is used here, based on the versioned G

; CHECK: vector.scevcheck:
; CHECK-NEXT: [[IDENT_CHECK:%.*]] = icmp ne i1 [[G]], true
; CHECK-NEXT: br i1 [[IDENT_CHECK]], label [[SCALAR_PH]], label [[VECTOR_PH:%.*]]
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