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[WebAssembly] Omit DBG_VALUE after terminator

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When a stackified variable has an associated `DBG_VALUE` instruction,
DebugFixup pass adds a `DBG_VALUE` instruction after the stackified
value's last use to clear the variable's debug range info. But when the
last use instruction is a terminator, it can cause a verification
failure (when run with `-verify-machineinstrs`) because there are no
instructions allowed after a terminator.

For example:
```
%myvar = ...
DBG_VALUE target-index(wasm-operand-stack), $noreg, !"myvar", ...
BR_IF 0, %myvar, ...
DBG_VALUE $noreg, $noreg, !"myvar", ...
```
In this test, `%myvar` is stackified, so the first `DBG_VALUE`
instruction's first operand has changed to `wasm-operand-stack` to
denote it. And an additional `DBG_VALUE` instruction is added after its
last use, `BR_IF`, to signal variable `myvar` is not in the operand
stack anymore. But because the `DBG_VALUE` instruction is added after
the `BR_IF`, a terminator, it fails MachineVerifier.

`DBG_VALUE` instructions are used in `DbgEntityHistoryCalculator` to
compute value ranges to emit DWARF info, and it turns out the
`DbgEntityHistoryCalculator` terminates ranges at the end of a BB, so we
don't need to emit `DBG_VALUE` after a terminator.

Fixes https://bugs.llvm.org/show_bug.cgi?id=50175.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D102309
This commit is contained in:
Heejin Ahn 2021-05-11 22:46:52 -07:00
parent 871c0f68dd
commit 5ec2c45efb
2 changed files with 38 additions and 8 deletions
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9
lib/Target/WebAssembly/WebAssemblyDebugFixup.cpp
View File

@ -111,14 +111,17 @@ bool WebAssemblyDebugFixup::runOnMachineFunction(MachineFunction &MF) {
Stack.pop_back();
assert(Prev.Reg == MO.getReg() &&
"WebAssemblyDebugFixup: Pop: Register not matched!");
if (Prev.DebugValue) {
// We should not put a DBG_VALUE after a terminator; debug ranges
// are terminated at the end of a BB anyway.
if (Prev.DebugValue && !MI.isTerminator()) {
// This stackified reg is a variable that started life at
// Prev.DebugValue, so now that we're popping it we must insert
// a $noreg DBG_VALUE for the variable to end it, right after
// the current instruction.
BuildMI(*Prev.DebugValue->getParent(), std::next(MII),
Prev.DebugValue->getDebugLoc(), TII->get(WebAssembly::DBG_VALUE), false,
Register(), Prev.DebugValue->getOperand(2).getMetadata(),
Prev.DebugValue->getDebugLoc(),
TII->get(WebAssembly::DBG_VALUE), false, Register(),
Prev.DebugValue->getOperand(2).getMetadata(),
Prev.DebugValue->getOperand(3).getMetadata());
}
}

37
test/CodeGen/WebAssembly/stackified-debug.ll
View File

@ -1,4 +1,5 @@
; RUN: llc < %s | FileCheck %s
; RUN: llc -verify-machineinstrs < %s | FileCheck %s
; RUN: llc -filetype=obj %s -o - | llvm-dwarfdump - | FileCheck %s --check-prefix DWARF
; Input C code:
@ -42,8 +43,6 @@
; CHECK: .int8 159 # DW_OP_stack_value
source_filename = "stackified.c"
target triple = "wasm32-unknown-unknown"
@ -57,10 +56,32 @@ entry:
ret void, !dbg !22
}
; DebugFixup pass should not add a DBG_VALUE after the BR_IF instruction at the
; end of the 'entry' BB, because it is not allowed to have more instructions
; after a terminator and debug ranges are terminated at the end of a BB anyway.
; If this passes 'llc -verify-machineinstrs', that means the DBG_VALUE
; instruction is correctly omitted.
; DWARF-LABEL: DW_AT_name ("no_dbg_value_after_terminator")
; DWARF: DW_TAG_variable
; DWARF-NEXT: DW_AT_location
; DWARF-NEXT: [
; DWARF-NEXT: DW_AT_name ("myvar")
define void @no_dbg_value_after_terminator(i32 %a, i32 %b) !dbg !23 {
entry:
%cmp = icmp ne i32 %a, %b, !dbg !25
call void @llvm.dbg.value(metadata i1 %cmp, metadata !27, metadata !DIExpression(DW_OP_LLVM_convert, 1, DW_ATE_unsigned, DW_OP_LLVM_convert, 8, DW_ATE_unsigned, DW_OP_stack_value)), !dbg !25
br i1 %cmp, label %bb.1, label %bb.0, !dbg !25
bb.0: ; preds = %entry
unreachable
bb.1: ; preds = %entry
ret void
}
declare i32 @input()
declare !dbg !4 void @output(i32, i32)
declare void @llvm.dbg.value(metadata, metadata, metadata)
!llvm.dbg.cu = !{!0}
@ -90,3 +111,9 @@ declare void @llvm.dbg.value(metadata, metadata, metadata)
!20 = !DILocation(line: 5, column: 11, scope: !12)
!21 = !DILocation(line: 6, column: 3, scope: !12)
!22 = !DILocation(line: 7, column: 1, scope: !12)
!23 = distinct !DISubprogram(name: "no_dbg_value_after_terminator", scope: null, type: !24, spFlags: DISPFlagDefinition, unit: !0)
!24 = !DISubroutineType(types: !2)
!25 = !DILocation(line: 0, scope: !26)
!26 = distinct !DILexicalBlock(scope: !23)
!27 = !DILocalVariable(name: "myvar", scope: !26, type: !28)
!28 = !DIBasicType(name: "bool", size: 8, encoding: DW_ATE_boolean)