[WebAssembly] Fix debug locations for ExplicitLocals pass

[aardappel]

As for the changes to DW_AT_low/high_pc the code generated for this test is:

000002 func[0] <foo>:
 000003: 01 7f                      | local[0] type=i32
 000005: 23 80 80 80 80 00          | global.get 0 <env.__stack_pointer>
 00000b: 41 10                      | i32.const 16
 00000d: 6b                         | i32.sub
 00000e: 22 01                      | local.tee 1
 000010: 20 00                      | local.get 0
 000012: 36 02 0c                   | i32.store 2 12
 000015: 20 01                      | local.get 1
 000017: 41 01                      | i32.const 1
 000019: 36 02 08                   | i32.store 2 8
 00001c: 20 01                      | local.get 1
 00001e: 41 02                      | i32.const 2
 000020: 36 02 04                   | i32.store 2 4
 000023: 20 01                      | local.get 1
 000025: 20 01                      | local.get 1
 000027: 28 02 04                   | i32.load 2 4
 00002a: 36 02 0c                   | i32.store 2 12
 00002d: 20 01                      | local.get 1
 00002f: 28 02 0c                   | i32.load 2 12
 000032: 20 01                      | local.get 1
 000034: 28 02 08                   | i32.load 2 8
 000037: 6a                         | i32.add
 000038: 0b                         | end

So it would seem the new offsets include the code from 0x23 to 0x34, which is the code the uses the alloca-ed vars, not the part that initializes them. But the old values end up pointing in the middle of an instruction? Was that ever correct?

[sbc100]

This change seems to have broken at least one emscripten test:

./tests/runner other.test_pthread_lsan_leak

Seems like the source map locations are off.

Maybe we can revert while we figure it out since there are other other recent llvm changes that we want to get into emscripten ASAP?

[sbc100]

In D112487#3097340, @sbc100 wrote:

This change seems to have broken at least one emscripten test:

./tests/runner other.test_pthread_lsan_leak

Seems like the source map locations are off.

Maybe we can revert while we figure it out since there are other other recent llvm changes that we want to get into emscripten ASAP?

Link to failing integration bot: https://logs.chromium.org/logs/emscripten-releases/buildbucket/cr-buildbucket/8832019855439718609/+/steps/Emscripten_testsuite__upstream__other_/0/stdout

llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp

for (MachineInstr &MI : llvm::make_early_inc_range(MBB)) {
           continue;
         }
 
         // Insert a local.get.
         unsigned LocalId = getLocalId(Reg2Local, MFI, CurLocal, OldReg);
         const TargetRegisterClass *RC = MRI.getRegClass(OldReg);
         Register NewReg = MRI.createVirtualRegister(RC);
         unsigned Opc = getLocalGetOpcode(RC);
+        // Use a blank DebugLoc, because InsertPt may be discontinuous from
+        // the usage of this value, causing non-linear stepping in the
+        // debugger or function entry points where variables aren't live yet.
+        // See crbug.com/1251909, crbug.com/1249745
+        DebugLoc DL;
         InsertPt =
-            BuildMI(MBB, InsertPt, MI.getDebugLoc(), TII->get(Opc), NewReg)
+            BuildMI(MBB, InsertPt, DL, TII->get(Opc), NewReg)
                 .addImm(LocalId);
         MO.setReg(NewReg);
         MFI.stackifyVReg(MRI, NewReg);
         Changed = true;
       }
 
       // Coalesce and eliminate COPY instructions.
       if (WebAssembly::isCopy(MI.getOpcode())) {

llvm/test/MC/WebAssembly/debug-localvar.ll

 ; CHECK-LABEL:     DW_TAG_variable
 ; CHECK-NEXT:                  DW_AT_location (DW_OP_fbreg +8)
 ; CHECK-NEXT:                  DW_AT_name ("a")
 ; CHECK-NEXT:                  DW_AT_decl_file  ("/s/llvm-upstream{{(/|\\)}}debugtest.c")
 ; CHECK-NEXT:                  DW_AT_decl_line  (2)
 ; CHECK-NEXT:                  DW_AT_type (0x00000073 "int")
 
 ; CHECK-LABEL:     DW_TAG_lexical_block
-; CHECK-NEXT:                  DW_AT_low_pc (0x0000001c)
-; CHECK-NEXT:                  DW_AT_high_pc  (0x0000002d)
+; CHECK-NEXT:                  DW_AT_low_pc (0x0000001e)
+; CHECK-NEXT:                  DW_AT_high_pc  (0x0000002f)
 
 ; CHECK-LABEL:       DW_TAG_variable
 ; CHECK-NEXT:                    DW_AT_location (DW_OP_fbreg +4)
 ; CHECK-NEXT:                    DW_AT_name ("b")
 ; CHECK-NEXT:                    DW_AT_decl_file  ("/s/llvm-upstream{{(/|\\)}}debugtest.c")
 ; CHECK-NEXT:                    DW_AT_decl_line  (4)
 ; CHECK-NEXT:                    DW_AT_type (0x00000073 "int")