[ThinLTO] Add funtions in callees metadata to CallGraphEdges


[ThinLTO] Add funtions in callees metadata to CallGraphEdges

If there's a callees metadata attached to the indirect call instruction, add CallGraphEdges to the callees mentioned in the metadata when computing FunctionSummary.

  • Why this is necessary:

Consider following code example:

static int f1(int x) {...}
static int f2(int x);
static int (*fptr)(int) = f2;
static int f2(int x) {
  if (x) fptr=f1; return f1(x);
int foo(int x) {
  (*fptr)(x); // !callees metadata of !{i32 (i32)* @f1, i32 (i32)* @f2} would be attached to this call.

int bar(int x) {
  return foo(x);

At LTO time when foo.o is imported into bar.o, function foo might be inlined into bar and PGO-guided indirect call promotion will run after that. If the profile data tells that the promotion of @f1 or @f2 is beneficial, the optimizer will check if the "promoted" @f1 or @f2 (such as @f1.llvm.0 or @f2.llvm.0) is available. Without this patch, importing !callees metadata would only add promoted declarations of @f1 and @f2 to the bar.o, but still the optimizer will assume that the function is available and perform the promotion. The result of that is link failure with undefined reference to @f1.llvm.0.

This patch fixes this problem by adding callees in the !callees metadata to CallGraphEdges so that their definition would be properly imported into.

One may ask that there already is a logic to add indirect call promotion targets to be added to CallGraphEdges. However, if profile data says "indirect call promotion is only beneficial under a certain inline context", the logic wouldn't work. In the code example above, if profile data is like

    1: foo:100000
        1: 100000 f1:100000

, Computing FunctionSummary for foo.o wouldn't add foo->f1 to CallGraphEdges. (Also, it is at least "possible" that one can provide profile data to only link step but not to compilation step).

Reviewers: tejohnson, mehdi_amini, pcc

Reviewed By: tejohnson

Subscribers: inglorion, eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D44399


twohMar 12 2018, 9:26 PM
Differential Revision
D44399: [ThinLTO] Add funtions in callees metadata to CallGraphEdges
rL327357: Use PLT relocations in test.