[PATCH] D42946: Verify profile data confirms large loop trip counts.
Mircea Trofin via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Mon Feb 5 21:00:35 PST 2018
mtrofin created this revision.
mtrofin added reviewers: twoh, mkuper.
Herald added a subscriber: llvm-commits.
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.
Repository:
rL LLVM
https://reviews.llvm.org/D42946
Files:
lib/Transforms/Vectorize/LoopVectorize.cpp
test/Transforms/LoopVectorize/tripcount.ll
Index: test/Transforms/LoopVectorize/tripcount.ll
===================================================================
--- test/Transforms/LoopVectorize/tripcount.ll
+++ test/Transforms/LoopVectorize/tripcount.ll
@@ -57,7 +57,7 @@
}
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 +84,30 @@
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
+}
!0 = !{!"function_entry_count", i64 100}
!1 = !{!"branch_weights", i32 100, i32 0}
Index: lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- lib/Transforms/Vectorize/LoopVectorize.cpp
+++ lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -208,7 +208,7 @@
"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."));
@@ -8350,7 +8350,10 @@
unsigned ExpectedTC = SE->getSmallConstantMaxTripCount(L);
bool HasExpectedTC = (ExpectedTC > 0);
- if (!HasExpectedTC && LoopVectorizeWithBlockFrequency) {
+ // ExpectedTC may be large because it's bound by a variable. Check
+ // profiling information to validate we should vectorize.
+ if ((!HasExpectedTC || ExpectedTC >= TinyTripCountVectorThreshold)
+ && LoopVectorizeWithBlockFrequency) {
auto EstimatedTC = getLoopEstimatedTripCount(L);
if (EstimatedTC) {
ExpectedTC = *EstimatedTC;
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