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[Coroutines] Add missing llvm.dbg.declare's to cover for more allocas

Authored by bruno on Nov 10 2020, 11:20 AM.

Description

[Coroutines] Add missing llvm.dbg.declare's to cover for more allocas

Tracking local variables across suspend points is still somewhat incomplete.
Consider this coroutine snippet:

resumable foo() {
  int x[10] = {};
  int a = 3;
  co_await std::experimental::suspend_always();
  a++;
  x[0] = 1;
  a += 2;
  x[1] = 2;
  a += 3;
  x[2] = 3;
}

Can't manage to print a or x if they turn out to be allocas during
CoroSplit (which happens if you build this code with -O0 prior to this
commit):

* thread #1, queue = 'com.apple.main-thread', stop reason = step over
    frame #0: 0x0000000100003729 main-noprint`foo() at main-noprint.cpp:43:5
   40     co_await std::experimental::suspend_always();
   41     a++;
   42     x[0] = 1;
-> 43     a += 2;
   44     x[1] = 2;
   45     a += 3;
   46     x[2] = 3;
(lldb) p x
error: <user expression 21>:1:1: use of undeclared identifier 'x'
x
^

The generated IR contains a llvm.dbg.declare for x in it's initialization
basic block. After CoroSplit, the llvm.dbg.declare might not dominate all of
x uses and we lose debugging quality.

Add llvm.dbg.values to all relevant basic blocks such that if later
transformations break the dominance the reliable debug info is already in
place. For instance, this BB:

await.ready:
  ...
  %arrayidx = getelementptr inbounds [10 x i32], [10 x i32]* %x.reload.addr, i64 0, i64 0, !dbg !760
  ...
  %arrayidx19 = getelementptr inbounds [10 x i32], [10 x i32]* %x.reload.addr, i64 0, i64 1, !dbg !763
  ...
  %arrayidx21 = getelementptr inbounds [10 x i32], [10 x i32]* %x.reload.addr, i64 0, i64 2, !dbg !766

becomes:

await.ready:
  ...
  call void @llvm.dbg.value(metadata [10 x i32]* %x.reload.addr, metadata !751, metadata !DIExpression()), !dbg !753
  ...
  %arrayidx = getelementptr inbounds [10 x i32], [10 x i32]* %x.reload.addr, i64 0, i64 0, !dbg !760
  ...
  %arrayidx19 = getelementptr inbounds [10 x i32], [10 x i32]* %x.reload.addr, i64 0, i64 1, !dbg !763
  ...
  %arrayidx21 = getelementptr inbounds [10 x i32], [10 x i32]* %x.reload.addr, i64 0, i64 2, !dbg !766

Differential Revision: https://reviews.llvm.org/D90772

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