[CSSPGO][Preinliner] Use one for zero-sized functions when computing inline budge

[hoy]

Summary

Use one to avoid multiplication with a zero func size which otherwise would result in no inlining. This could happen when all of the original function's code is gone and only inlined code is left.

Testing on one internal benchmark shows neutral perf impact, but dynamic instruction count reduces by 0.5%. Also there are visible differences in generated profile.

[wlei]

I think we actually will use the ProfileInlineLimitMin which is 100 for the count, see SizeLimit = std::max(SizeLimit, (unsigned)ProfileInlineLimitMin);

ProfileInlineGrowthLimit is by default set to 12, and 1 * 12 is still smaller than 100.

[hoy]

In D138231#3934734, @wlei wrote:

I think we actually will use the ProfileInlineLimitMin which is 100 for the count, see SizeLimit = std::max(SizeLimit, (unsigned)ProfileInlineLimitMin);

ProfileInlineGrowthLimit is by default set to 12, and 1 * 12 is still smaller than 100.

Oh yes, you are right. This change is supposed to be on top of another change which replaces ProfileInlineLimitMin with a multiplifier ProfileInlineGrowthLimitMin.

Abandoning this for now until rebased.

llvm/tools/llvm-profgen/CSPreInliner.cpp

 void CSPreInliner::processFunction(const StringRef Name) {
   FunctionSamples *FSamples = ContextTracker.getBaseSamplesFor(Name);
   if (!FSamples)
     return;
 
   unsigned FuncSize =
       getFuncSize(ContextTracker.getContextNodeForProfile(FSamples));
   unsigned FuncFinalSize = FuncSize;
-  unsigned SizeLimit = FuncSize * ProfileInlineGrowthLimit;
+  // Use one to avoid multiplication with a zero func size. This could happen
+  // when all of the original function's code is gone and only inlined code is
+  // left.
+  unsigned ModifiedFuncSize = FuncSize ? FuncSize : 1;
+  unsigned SizeLimit = ModifiedFuncSize * ProfileInlineGrowthLimit;
   SizeLimit = std::min(SizeLimit, (unsigned)ProfileInlineLimitMax);
   SizeLimit = std::max(SizeLimit, (unsigned)ProfileInlineLimitMin);
 
   LLVM_DEBUG(dbgs() << "Process " << Name
                     << " for context-sensitive pre-inlining (pre-inline size: "
                     << FuncSize << ", size limit: " << SizeLimit << ")\n");
 
   ProfiledCandidateQueue CQueue;