[clang] [lld] [llvm] Reland "Compute GUIDs once and store in metadata" (#184065) (PR #201849)

[Mircea Trofin via cfe-commits]

mtrofin wrote:

Ah, ok. About that one: `perf stat -e instructions -r 10` actually shows ~6% _improvement_ with the new change; when I look at `cycles`I see a 1% improvement. Wallclock average showed the same. But, the wallclock measurements are actually pretty noisy - as much as 1.31% over the 10 reps.

Looking at the profile itself, (`perf record -e instructions`):

Before:

```
+   13.23%    13.23%  ld.lld          lld                   [.] llvm::MD5::body(llvm::ArrayRef<unsigne
+   12.83%    12.82%  ld.lld          lld                   [.] llvm::SimpleBitstreamCursor::Read(unsi
+   10.19%    10.19%  ld.lld          lld                   [.] llvm::SimpleBitstreamCursor::ReadVBR64
+    6.54%     0.00%  ld.lld          [unknown]             [.] 0000000000000000
+    4.64%     4.64%  ld.lld          lld                   [.] llvm::BitstreamCursor::readRecord(unsi
+    2.11%     2.11%  ld.lld          lld                   [.] llvm::xxh3_64bits(unsigned char const*
```

After:

```
+   15.02%    15.02%  ld.lld          lld                   [.] llvm::SimpleBitstreamCursor::Read(unsi
+   10.64%    10.64%  ld.lld          lld                   [.] llvm::SimpleBitstreamCursor::ReadVBR64
+    8.56%     0.00%  ld.lld          [unknown]             [.] 0000000000000000
+    6.48%     6.48%  ld.lld          lld                   [.] llvm::BitstreamCursor::readRecord(unsi
+    3.19%     3.19%  ld.lld          lld                   [.] llvm::MD5::body(llvm::ArrayRef<unsigne
+    2.96%     2.96%  ld.lld          lld                   [.] llvm::xxh3_64bits(unsigned char const*
```

This lines up with the "we serialize more" (and so we deserialize during linking).  
  
Not sure what to make of the `perf stat` results difference. Perhaps  cold runs -> cold caches ->  patch is deserialization heavy -> deserialization (of the GUIDs) costs more than MD5 hashing, but then (hot caches) "the tables flip".

https://github.com/llvm/llvm-project/pull/201849