[CSSPGO][Preinliner] Always inline zero-sized functions.

[hoy]

Summary

Zero-sized functions should be cost-free in term of size budget, so they should be considered during inlining even if we run out of size budget.

This appears to give 0.5% win for one of our internal services.

[wenlei]

Can this be achieved by simply tweaking ProfiledCandidateComparer so zero SizeCost candidate always comes before non-zero ones?

[hoy]

In D153820#4453775, @wenlei wrote:

Can this be achieved by simply tweaking ProfiledCandidateComparer so zero SizeCost candidate always comes before non-zero ones?

Good idea!

[hoy]

Updating D153820: [CSSPGO][Preinliner] Always inline zero-sized functions.

[wenlei]

lgtm, thanks.

llvm/tools/llvm-profgen/CSPreInliner.h

47–48

nit: merge the two ifs into a single line?

if ((LHS.SizeCost == 0 || RHS.SizeCost == 0) && LHS.SizeCost != RHS.SizeCost)
    return return RHS.SizeCost == 0;

[hoy]

Updating D153820: [CSSPGO][Preinliner] Always inline zero-sized functions.

llvm/tools/llvm-profgen/CSPreInliner.h

struct ProfiledInlineCandidate {
   // Size proxy for function under particular call context.
   uint64_t SizeCost;
 };
 
 // Inline candidate comparer using call site weight
 struct ProfiledCandidateComparer {
   bool operator()(const ProfiledInlineCandidate &LHS,
                   const ProfiledInlineCandidate &RHS) {
+    // Always prioritize inlining zero-sized functions as they do not affect the
+    // size budget. This could happen when all of the callee's code is gone and
+    // only pseudo probes are left.
+    if ((LHS.SizeCost == 0 || RHS.SizeCost == 0) &&
+        (LHS.SizeCost != RHS.SizeCost))
+      return RHS.SizeCost == 0;
+
     if (LHS.CallsiteCount != RHS.CallsiteCount)
       return LHS.CallsiteCount < RHS.CallsiteCount;
 
     if (LHS.SizeCost != RHS.SizeCost)
       return LHS.SizeCost > RHS.SizeCost;
 
     // Tie breaker using GUID so we have stable/deterministic inlining order
     assert(LHS.CalleeSamples && RHS.CalleeSamples &&