Verify profile data confirms large loop trip counts.

Summary:
Loops with inequality comparers, such as:

   // unsigned bound
   for (unsigned i = 1; i < bound; ++i) {...}

have getSmallConstantMaxTripCount report a large maximum static
trip count - in this case, 0xffff fffe. However, profiling info
may show that the trip count is much smaller, and thus
counter-recommend vectorization.

This change:
- flips loop-vectorize-with-block-frequency on by default.
- validates profiled loop frequency data supports vectorization,
  when static info appears to not counter-recommend it. Absence
  of profile data means we rely on static data, just as we've
  done so far.

Reviewers: twoh, mkuper, davidxl, tejohnson, Ayal

Reviewed By: davidxl

Subscribers: bkramer, llvm-commits

Differential Revision: https://reviews.llvm.org/D42946

llvm-svn: 324543
This commit is contained in:
Mircea Trofin 2018-02-07 23:29:52 +00:00
parent f39cd6d76b
commit 239ce88ecb
2 changed files with 141 additions and 5 deletions

View File

@ -208,7 +208,7 @@ static cl::opt<unsigned> SmallLoopCost(
"The cost of a loop that is considered 'small' by the interleaver."));
static cl::opt<bool> LoopVectorizeWithBlockFrequency(
"loop-vectorize-with-block-frequency", cl::init(false), cl::Hidden,
"loop-vectorize-with-block-frequency", cl::init(true), cl::Hidden,
cl::desc("Enable the use of the block frequency analysis to access PGO "
"heuristics minimizing code growth in cold regions and being more "
"aggressive in hot regions."));
@ -8347,9 +8347,21 @@ bool LoopVectorizePass::processLoop(Loop *L) {
// Check the loop for a trip count threshold: vectorize loops with a tiny trip
// count by optimizing for size, to minimize overheads.
unsigned ExpectedTC = SE->getSmallConstantMaxTripCount(L);
bool HasExpectedTC = (ExpectedTC > 0);
// Prefer constant trip counts over profile data, over upper bound estimate.
unsigned ExpectedTC = 0;
bool HasExpectedTC = false;
if (const SCEVConstant *ConstExits =
dyn_cast<SCEVConstant>(SE->getBackedgeTakenCount(L))) {
const APInt &ExitsCount = ConstExits->getAPInt();
// We are interested in small values for ExpectedTC. Skip over those that
// can't fit an unsigned.
if (ExitsCount.ult(std::numeric_limits<unsigned>::max())) {
ExpectedTC = static_cast<unsigned>(ExitsCount.getZExtValue()) + 1;
HasExpectedTC = true;
}
}
// ExpectedTC may be large because it's bound by a variable. Check
// profiling information to validate we should vectorize.
if (!HasExpectedTC && LoopVectorizeWithBlockFrequency) {
auto EstimatedTC = getLoopEstimatedTripCount(L);
if (EstimatedTC) {
@ -8357,6 +8369,10 @@ bool LoopVectorizePass::processLoop(Loop *L) {
HasExpectedTC = true;
}
}
if (!HasExpectedTC) {
ExpectedTC = SE->getSmallConstantMaxTripCount(L);
HasExpectedTC = (ExpectedTC > 0);
}
if (HasExpectedTC && ExpectedTC < TinyTripCountVectorThreshold) {
DEBUG(dbgs() << "LV: Found a loop with a very small trip count. "

View File

@ -57,7 +57,7 @@ for.end: ; preds = %for.body
}
define i32 @foo_low_trip_count3(i1 %cond, i32 %bound) !prof !0 {
; The loop has low invocation count compare to the function invocation count,
; The loop has low invocation count compare to the function invocation count,
; but has a high trip count per invocation. Vectorize it.
; CHECK-LABEL: @foo_low_trip_count3(
@ -84,6 +84,126 @@ for.end: ; preds = %for.body
ret i32 0
}
define i32 @foo_low_trip_count_icmp_sgt(i32 %bound) {
; Simple loop with low tripcount and inequality test for exit.
; Should not be vectorized.
; CHECK-LABEL: @foo_low_trip_count_icmp_sgt(
; CHECK-NOT: <{{[0-9]+}} x i8>
entry:
br label %for.body
for.body: ; preds = %for.body, %entry
%i.08 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
%arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @tab, i32 0, i32 %i.08
%0 = load i8, i8* %arrayidx, align 1
%cmp1 = icmp eq i8 %0, 0
%. = select i1 %cmp1, i8 2, i8 1
store i8 %., i8* %arrayidx, align 1
%inc = add nsw i32 %i.08, 1
%exitcond = icmp sgt i32 %i.08, %bound
br i1 %exitcond, label %for.end, label %for.body, !prof !1
for.end: ; preds = %for.body
ret i32 0
}
define i32 @const_low_trip_count() {
; Simple loop with constant, small trip count and no profiling info.
; CHECK-LABEL: @const_low_trip_count
; CHECK-NOT: <{{[0-9]+}} x i8>
entry:
br label %for.body
for.body: ; preds = %for.body, %entry
%i.08 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
%arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @tab, i32 0, i32 %i.08
%0 = load i8, i8* %arrayidx, align 1
%cmp1 = icmp eq i8 %0, 0
%. = select i1 %cmp1, i8 2, i8 1
store i8 %., i8* %arrayidx, align 1
%inc = add nsw i32 %i.08, 1
%exitcond = icmp slt i32 %i.08, 2
br i1 %exitcond, label %for.body, label %for.end
for.end: ; preds = %for.body
ret i32 0
}
define i32 @const_large_trip_count() {
; Simple loop with constant large trip count and no profiling info.
; CHECK-LABEL: @const_large_trip_count
; CHECK: <{{[0-9]+}} x i8>
entry:
br label %for.body
for.body: ; preds = %for.body, %entry
%i.08 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
%arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @tab, i32 0, i32 %i.08
%0 = load i8, i8* %arrayidx, align 1
%cmp1 = icmp eq i8 %0, 0
%. = select i1 %cmp1, i8 2, i8 1
store i8 %., i8* %arrayidx, align 1
%inc = add nsw i32 %i.08, 1
%exitcond = icmp slt i32 %i.08, 1000
br i1 %exitcond, label %for.body, label %for.end
for.end: ; preds = %for.body
ret i32 0
}
define i32 @const_small_trip_count_step() {
; Simple loop with static, small trip count and no profiling info.
; CHECK-LABEL: @const_small_trip_count_step
; CHECK-NOT: <{{[0-9]+}} x i8>
entry:
br label %for.body
for.body: ; preds = %for.body, %entry
%i.08 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
%arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @tab, i32 0, i32 %i.08
%0 = load i8, i8* %arrayidx, align 1
%cmp1 = icmp eq i8 %0, 0
%. = select i1 %cmp1, i8 2, i8 1
store i8 %., i8* %arrayidx, align 1
%inc = add nsw i32 %i.08, 5
%exitcond = icmp slt i32 %i.08, 10
br i1 %exitcond, label %for.body, label %for.end
for.end: ; preds = %for.body
ret i32 0
}
define i32 @const_trip_over_profile() {
; constant trip count takes precedence over profile data
; CHECK-LABEL: @const_trip_over_profile
; CHECK: <{{[0-9]+}} x i8>
entry:
br label %for.body
for.body: ; preds = %for.body, %entry
%i.08 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
%arrayidx = getelementptr inbounds [32 x i8], [32 x i8]* @tab, i32 0, i32 %i.08
%0 = load i8, i8* %arrayidx, align 1
%cmp1 = icmp eq i8 %0, 0
%. = select i1 %cmp1, i8 2, i8 1
store i8 %., i8* %arrayidx, align 1
%inc = add nsw i32 %i.08, 1
%exitcond = icmp slt i32 %i.08, 1000
br i1 %exitcond, label %for.body, label %for.end, !prof !1
for.end: ; preds = %for.body
ret i32 0
}
!0 = !{!"function_entry_count", i64 100}
!1 = !{!"branch_weights", i32 100, i32 0}