Add minimum count threshold for indirect call promotion

This adds a new command-line option -icp-minimum-count-threshold to prevent
promotion of indirect call candidates with very low absolute counts, even
if they meet the percentage-based profitability thresholds.

Currently, candidates with counts as low as 1-2 calls can be promoted if
they represent a significant percentage of the call site. This can lead to
unprofitable micro-optimizations that increase code size without meaningful
performance benefits.

The new threshold defaults to 0 to maintain backward compatibility but can
be set to values like 5-20 for production use to filter out statistically
insignificant candidates.

Added comprehensive test coverage for the new threshold including negative
tests (blocking low-count promotions), positive tests (allowing high-count
promotions), and edge cases (threshold exactly at count value).

Author: snehasish

llvm/lib/Analysis/IndirectCallPromotionAnalysis.cpp

@@ -37,6 +37,12 @@ static cl::opt<unsigned>
                              cl::desc("The percentage threshold against total "
                                       "count for the promotion"));
 
+// Set the minimum absolute count threshold for indirect call promotion.
+// Candidates with counts below this threshold will not be promoted.
+static cl::opt<unsigned> ICPMinimumCountThreshold(
+    "icp-minimum-count-threshold", cl::init(0), cl::Hidden,
+    cl::desc("Minimum absolute count for promotion candidate"));
+
 // Set the maximum number of targets to promote for a single indirect-call
 // callsite.
 static cl::opt<unsigned>
@@ -51,7 +57,8 @@ cl::opt<unsigned> MaxNumVTableAnnotations(
 bool ICallPromotionAnalysis::isPromotionProfitable(uint64_t Count,
                                                    uint64_t TotalCount,
                                                    uint64_t RemainingCount) {
-  return Count * 100 >= ICPRemainingPercentThreshold * RemainingCount &&
+  return Count >= ICPMinimumCountThreshold &&
+         Count * 100 >= ICPRemainingPercentThreshold * RemainingCount &&
          Count * 100 >= ICPTotalPercentThreshold * TotalCount;
 }
 

llvm/test/Transforms/PGOProfile/indirect_call_promotion.ll

@@ -1,6 +1,12 @@
 ; RUN: opt < %s -passes=pgo-icall-prom -S -icp-total-percent-threshold=50 | FileCheck %s --check-prefix=ICALL-PROM
 ; RUN: opt < %s -passes=pgo-icall-prom -S -pass-remarks=pgo-icall-prom -icp-remaining-percent-threshold=0 -icp-total-percent-threshold=0 -icp-max-prom=4 2>&1 | FileCheck %s --check-prefix=PASS-REMARK
 ; RUN: opt < %s -passes=pgo-icall-prom -S -pass-remarks=pgo-icall-prom -icp-remaining-percent-threshold=0 -icp-total-percent-threshold=20 -icp-max-prom=4 2>&1 | FileCheck %s --check-prefix=PASS2-REMARK
+; Test minimum count threshold - should prevent func1 promotion (count 10 < threshold 15)
+; RUN: opt < %s -passes=pgo-icall-prom -S -pass-remarks=pgo-icall-prom -icp-minimum-count-threshold=15 -icp-remaining-percent-threshold=0 -icp-total-percent-threshold=0 -icp-max-prom=4 2>&1 | FileCheck %s --check-prefix=MIN-COUNT-BLOCK
+; Test minimum count threshold - should allow func4 promotion (count 1030 > threshold 15) 
+; RUN: opt < %s -passes=pgo-icall-prom -S -pass-remarks=pgo-icall-prom -icp-minimum-count-threshold=15 -icp-remaining-percent-threshold=0 -icp-total-percent-threshold=0 -icp-max-prom=4 2>&1 | FileCheck %s --check-prefix=MIN-COUNT-ALLOW
+; Test edge case - threshold exactly at count value
+; RUN: opt < %s -passes=pgo-icall-prom -S -pass-remarks=pgo-icall-prom -icp-minimum-count-threshold=10 -icp-remaining-percent-threshold=0 -icp-total-percent-threshold=0 -icp-max-prom=4 2>&1 | FileCheck %s --check-prefix=MIN-COUNT-EDGE
 
 ; PASS-REMARK: remark: <unknown>:0:0: Promote indirect call to func4 with count 1030 out of 1600
 ; PASS-REMARK: remark: <unknown>:0:0: Promote indirect call to func2 with count 410 out of 570
@@ -12,6 +18,21 @@
 ; PASS2-REMARK-NOT: remark: <unknown>:0:0: Promote indirect call to func3
 ; PASS2-REMARK-NOT: remark: <unknown>:0:0: Promote indirect call to func1
 
+; MIN-COUNT-BLOCK: remark: <unknown>:0:0: Promote indirect call to func4 with count 1030 out of 1600
+; MIN-COUNT-BLOCK: remark: <unknown>:0:0: Promote indirect call to func2 with count 410 out of 570
+; MIN-COUNT-BLOCK: remark: <unknown>:0:0: Promote indirect call to func3 with count 150 out of 160
+; MIN-COUNT-BLOCK-NOT: remark: <unknown>:0:0: Promote indirect call to func1
+
+; MIN-COUNT-ALLOW: remark: <unknown>:0:0: Promote indirect call to func4 with count 1030 out of 1600
+; MIN-COUNT-ALLOW: remark: <unknown>:0:0: Promote indirect call to func2 with count 410 out of 570
+; MIN-COUNT-ALLOW: remark: <unknown>:0:0: Promote indirect call to func3 with count 150 out of 160
+; MIN-COUNT-ALLOW-NOT: remark: <unknown>:0:0: Promote indirect call to func1
+
+; MIN-COUNT-EDGE: remark: <unknown>:0:0: Promote indirect call to func4 with count 1030 out of 1600
+; MIN-COUNT-EDGE: remark: <unknown>:0:0: Promote indirect call to func2 with count 410 out of 570
+; MIN-COUNT-EDGE: remark: <unknown>:0:0: Promote indirect call to func3 with count 150 out of 160
+; MIN-COUNT-EDGE: remark: <unknown>:0:0: Promote indirect call to func1 with count 10 out of 10
+
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"