[BOLT][merge-fdata] Initialize YAML profile header (llvm#109613)

While merging profiles, some fields in the input header, e.g.
HashFunction, could be uninitialized leading to a UMR. Initialize merged
header with the first input header.

Fixes llvm#109592

Author: maksfb

bolt/test/merge-fdata-uninitialized-header.test

@@ -0,0 +1,45 @@
+## Test that merge-fdata correctly handles YAML header with an uninitialized
+## fields. a.yaml does not have hash-func set and it used to crash merge-fdata.
+
+# REQUIRES: system-linux
+
+# RUN: split-file %s %t
+# RUN: not merge-fdata %t/a.yaml %t/b.yaml 2>&1 | FileCheck %s
+
+# CHECK: cannot merge profiles with different hash functions
+
+#--- a.yaml
+---
+header:
+  profile-version: 1
+  binary-name:     'a.out'
+  binary-build-id: '<unknown>'
+  profile-flags:   [ lbr ]
+  profile-origin:  branch profile reader
+  profile-events:  ''
+  dfs-order:       false
+functions:
+  - name:            'main'
+    fid:             1
+    hash:            0x50BBA3441D436491
+    exec:            1
+    nblocks:         0
+...
+#--- b.yaml
+---
+header:
+  profile-version: 1
+  binary-name:     'a.out'
+  binary-build-id: '<unknown>'
+  profile-flags:   [ lbr ]
+  profile-origin:  branch profile reader
+  profile-events:  ''
+  dfs-order:       false
+  hash-func: xxh3
+functions:
+  - name:            'main'
+    fid:             1
+    hash:            0x50BBA3441D436491
+    exec:            1
+    nblocks:         0
+...

bolt/tools/merge-fdata/merge-fdata.cpp

@@ -145,6 +145,10 @@ void mergeProfileHeaders(BinaryProfileHeader &MergedHeader,
     errs() << "WARNING: merging profiles with different sampling events\n";
     MergedHeader.EventNames += "," + Header.EventNames;
   }
+
+  if (MergedHeader.HashFunction != Header.HashFunction)
+    report_error("merge conflict",
+                 "cannot merge profiles with different hash functions");
 }
 
 void mergeBasicBlockProfile(BinaryBasicBlockProfile &MergedBB,
@@ -386,6 +390,7 @@ int main(int argc, char **argv) {
   // Merged information for all functions.
   StringMap<BinaryFunctionProfile> MergedBFs;
 
+  bool FirstHeader = true;
   for (std::string &InputDataFilename : Inputs) {
     ErrorOr<std::unique_ptr<MemoryBuffer>> MB =
         MemoryBuffer::getFileOrSTDIN(InputDataFilename);
@@ -409,7 +414,12 @@ int main(int argc, char **argv) {
     }
 
     // Merge the header.
-    mergeProfileHeaders(MergedHeader, BP.Header);
+    if (FirstHeader) {
+      MergedHeader = BP.Header;
+      FirstHeader = false;
+    } else {
+      mergeProfileHeaders(MergedHeader, BP.Header);
+    }
 
     // Do the function merge.
     for (BinaryFunctionProfile &BF : BP.Functions) {