[InstCombine] Fold @llvm.experimental.get.vector.length when cnt <= max_lanes (#169293)

On RISC-V, some loops that the loop vectorizer vectorizes pre-LTO may
turn out to have the exact trip count exposed after LTO, see #164762.

If the trip count is small enough we can fold away the
@llvm.experimental.get.vector.length intrinsic based on this corollary
from the LangRef:

> If %cnt is less than or equal to %max_lanes, the return value is equal
to %cnt.

This on its own doesn't remove the @llvm.experimental.get.vector.length
in #164762 since we also need to teach computeKnownBits about
@llvm.experimental.get.vector.length and the sub recurrence, but this PR
is a starting point.

I've added this in InstCombine rather than InstSimplify since we may
need to insert a truncation (@llvm.experimental.get.vector.length can
take an i64 %cnt argument, the result is always i32).

Note that there was something similar done in VPlan in #167647 for when
the loop vectorizer knows the trip count.

Author: lukel97

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -4016,6 +4016,27 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     }
     break;
   }
+  case Intrinsic::experimental_get_vector_length: {
+    // get.vector.length(Cnt, MaxLanes) --> Cnt when Cnt <= MaxLanes
+    unsigned BitWidth =
+        std::max(II->getArgOperand(0)->getType()->getScalarSizeInBits(),
+                 II->getType()->getScalarSizeInBits());
+    ConstantRange Cnt =
+        computeConstantRangeIncludingKnownBits(II->getArgOperand(0), false,
+                                               SQ.getWithInstruction(II))
+            .zextOrTrunc(BitWidth);
+    ConstantRange MaxLanes = cast<ConstantInt>(II->getArgOperand(1))
+                                 ->getValue()
+                                 .zextOrTrunc(Cnt.getBitWidth());
+    if (cast<ConstantInt>(II->getArgOperand(2))->isOne())
+      MaxLanes = MaxLanes.multiply(
+          getVScaleRange(II->getFunction(), Cnt.getBitWidth()));
+
+    if (Cnt.icmp(CmpInst::ICMP_ULE, MaxLanes))
+      return replaceInstUsesWith(
+          *II, Builder.CreateZExtOrTrunc(II->getArgOperand(0), II->getType()));
+    return nullptr;
+  }
   default: {
     // Handle target specific intrinsics
     std::optional<Instruction *> V = targetInstCombineIntrinsic(*II);

llvm/test/Transforms/InstCombine/get_vector_length.ll

@@ -0,0 +1,89 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt < %s -passes=instcombine,verify -S | FileCheck %s
+
+define i32 @cnt_known_lt() {
+; CHECK-LABEL: define i32 @cnt_known_lt() {
+; CHECK-NEXT:    ret i32 1
+;
+  %x = call i32 @llvm.experimental.get.vector.length(i32 1, i32 2, i1 false)
+  ret i32 %x
+}
+
+define i32 @cnt_not_known_lt() {
+; CHECK-LABEL: define i32 @cnt_not_known_lt() {
+; CHECK-NEXT:    [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 2, i32 1, i1 false)
+; CHECK-NEXT:    ret i32 [[X]]
+;
+  %x = call i32 @llvm.experimental.get.vector.length(i32 2, i32 1, i1 false)
+  ret i32 %x
+}
+
+define i32 @cnt_known_lt_scalable() vscale_range(2, 4) {
+; CHECK-LABEL: define i32 @cnt_known_lt_scalable(
+; CHECK-SAME: ) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:    ret i32 2
+;
+  %x = call i32 @llvm.experimental.get.vector.length(i32 2, i32 1, i1 true)
+  ret i32 %x
+}
+
+define i32 @cnt_not_known_lt_scalable() {
+; CHECK-LABEL: define i32 @cnt_not_known_lt_scalable() {
+; CHECK-NEXT:    [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 2, i32 1, i1 true)
+; CHECK-NEXT:    ret i32 [[X]]
+;
+  %x = call i32 @llvm.experimental.get.vector.length(i32 2, i32 1, i1 true)
+  ret i32 %x
+}
+
+define i32 @cnt_known_lt_runtime(i32 %x) {
+; CHECK-LABEL: define i32 @cnt_known_lt_runtime(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT:    [[ICMP:%.*]] = icmp ult i32 [[X]], 4
+; CHECK-NEXT:    call void @llvm.assume(i1 [[ICMP]])
+; CHECK-NEXT:    ret i32 [[X]]
+;
+  %icmp = icmp ule i32 %x, 3
+  call void @llvm.assume(i1 %icmp)
+  %y = call i32 @llvm.experimental.get.vector.length(i32 %x, i32 3, i1 false)
+  ret i32 %y
+}
+
+define i32 @cnt_known_lt_runtime_trunc(i64 %x) {
+; CHECK-LABEL: define i32 @cnt_known_lt_runtime_trunc(
+; CHECK-SAME: i64 [[X:%.*]]) {
+; CHECK-NEXT:    [[ICMP:%.*]] = icmp ult i64 [[X]], 4
+; CHECK-NEXT:    call void @llvm.assume(i1 [[ICMP]])
+; CHECK-NEXT:    [[Y:%.*]] = trunc nuw nsw i64 [[X]] to i32
+; CHECK-NEXT:    ret i32 [[Y]]
+;
+  %icmp = icmp ule i64 %x, 3
+  call void @llvm.assume(i1 %icmp)
+  %y = call i32 @llvm.experimental.get.vector.length(i64 %x, i32 3, i1 false)
+  ret i32 %y
+}
+
+; FIXME: We should be able to deduce the constant range from AssumptionCache
+; rather than relying on KnownBits, which in this case only knows x <= 3.
+define i32 @cnt_known_lt_runtime_assumption(i32 %x) {
+; CHECK-LABEL: define i32 @cnt_known_lt_runtime_assumption(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT:    [[ICMP:%.*]] = icmp ult i32 [[X]], 3
+; CHECK-NEXT:    call void @llvm.assume(i1 [[ICMP]])
+; CHECK-NEXT:    [[Y:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 [[X]], i32 2, i1 false)
+; CHECK-NEXT:    ret i32 [[Y]]
+;
+  %icmp = icmp ule i32 %x, 2
+  call void @llvm.assume(i1 %icmp)
+  %y = call i32 @llvm.experimental.get.vector.length(i32 %x, i32 2, i1 false)
+  ret i32 %y
+}
+
+
+define i32 @cnt_known_lt_i16() {
+; CHECK-LABEL: define i32 @cnt_known_lt_i16() {
+; CHECK-NEXT:    ret i32 1
+;
+  %x = call i32 @llvm.experimental.get.vector.length(i16 1, i32 2, i1 false)
+  ret i32 %x
+}