[PATCH] D92013: [DebugInfo] Don't use DW_OP_implicit_value for fragments

Tue Nov 24 01:49:20 PST 2020

labath created this revision.
labath added reviewers: dblaikie, probinson, aprantl, SouraVX.
Herald added a subscriber: hiraditya.
Herald added a project: LLVM.
labath requested review of this revision.

Currently using DW_OP_implicit_value in fragments produces invalid DWARF
expressions. (Such a case can occur in complex floats, for example.)

This problem manifests itself as a missing DW_OP_piece operation after
the last fragment. This happens because the function for printing
constant float value skips printing the accompanying DWARF expression,
as that would also print DW_OP_stack_value (which is not desirable in
this case). However, this also results in DW_OP_piece being skipped.

The reason that DW_OP_piece is missing only for the last piece is that
the act of printing the next fragment corrects this. However, it does
that for the wrong reason -- the code emitting this DW_OP_piece thinks
that the previous fragment was missing, and so it thinks that it needs
to skip over it in order to be able to print itself.

In a simple scenario this works out, but it's likely that in a more
complex setup (where some pieces are in fact missing), this logic would
go badly wrong. In a simple setup gdb also seems to not mind the fact
that the DW_OP_piece is missing, but it would also likely not handle
more complex use cases.

For this reason, this patch disables the usage of DW_OP_implicit_value
in the frament scenario (we will use DW_OP_const*** instead), until we
figure out the right way to deal with this. This guarantees that we
produce valid expressions, and gdb can handle both kinds of inputs
anyway.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D92013

Files:
  llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
  llvm/test/DebugInfo/X86/implicit_value-double.ll


Index: llvm/test/DebugInfo/X86/implicit_value-double.ll
===================================================================

--- llvm/test/DebugInfo/X86/implicit_value-double.ll
+++ llvm/test/DebugInfo/X86/implicit_value-double.ll
@@ -1,8 +1,8 @@
 ;; This test checks for emission of DW_OP_implicit_value operation
 ;; for double type.
 
-; RUN: llc -debugger-tune=gdb -filetype=obj %s -o -  | llvm-dwarfdump - | FileCheck %s
-; RUN: llc -debugger-tune=lldb -filetype=obj %s -o - | llvm-dwarfdump - | FileCheck %s
+; RUN: llc -O0 -debugger-tune=gdb -filetype=obj %s -o -  | llvm-dwarfdump - | FileCheck %s --check-prefixes=CHECK,BOTH
+; RUN: llc -O0 -debugger-tune=lldb -filetype=obj %s -o - | llvm-dwarfdump - | FileCheck %s --check-prefixes=CHECK,BOTH
 
 ; CHECK: .debug_info contents:
 ; CHECK: DW_TAG_variable
@@ -10,7 +10,7 @@
 ; CHECK-NEXT:                     [{{.*}}): DW_OP_implicit_value 0x8 0x1f 0x85 0xeb 0x51 0xb8 0x1e 0x09 0x40)
 ; CHECK-NEXT:  DW_AT_name    ("d")
 
-; RUN: llc -debugger-tune=sce -filetype=obj %s -o -  | llvm-dwarfdump - | FileCheck %s -check-prefix=SCE-CHECK
+; RUN: llc -O0 -debugger-tune=sce -filetype=obj %s -o -  | llvm-dwarfdump - | FileCheck %s -check-prefixes=SCE-CHECK,BOTH
 
 ; SCE-CHECK: .debug_info contents:
 ; SCE-CHECK: DW_TAG_variable
@@ -18,13 +18,11 @@
 ; SCE-CHECK-NEXT:                     [{{.*}}): DW_OP_constu 0x40091eb851eb851f, DW_OP_stack_value)
 ; SCE-CHECK-NEXT:  DW_AT_name    ("d")
 
-;; Generated from: clang -ggdb -O1
-;;int main() {
-;;        double d = 3.14;
-;;        printf("dummy\n");
-;;        d *= d;
-;;        return 0;
-;;}
+;; Using DW_OP_implicit_value for fragments is not currently supported.
+; BOTH: DW_TAG_variable
+; BOTH-NEXT:  DW_AT_location        ({{.*}}
+; BOTH-NEXT:                     [{{.*}}): DW_OP_constu 0x4047800000000000, DW_OP_stack_value, DW_OP_piece 0x8, DW_OP_constu 0x4052800000000000, DW_OP_stack_value, DW_OP_piece 0x8)
+; BOTH-NEXT:  DW_AT_name    ("c")
 
 ; ModuleID = 'implicit_value-double.c'
 source_filename = "implicit_value-double.c"
@@ -37,6 +35,8 @@
 define dso_local i32 @main() local_unnamed_addr #0 !dbg !7 {
 entry:
   call void @llvm.dbg.value(metadata double 3.140000e+00, metadata !12, metadata !DIExpression()), !dbg !14
+  call void @llvm.dbg.value(metadata double 4.700000e+01, metadata !17, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 64)), !dbg !14
+  call void @llvm.dbg.value(metadata double 7.400000e+01, metadata !17, metadata !DIExpression(DW_OP_LLVM_fragment, 64, 64)), !dbg !14
   %puts = call i32 @puts(i8* nonnull dereferenceable(1) getelementptr inbounds ([6 x i8], [6 x i8]* @str, i64 0, i64 0)), !dbg !15
   call void @llvm.dbg.value(metadata double undef, metadata !12, metadata !DIExpression()), !dbg !14
   ret i32 0, !dbg !16
@@ -67,9 +67,11 @@
 !8 = !DISubroutineType(types: !9)
 !9 = !{!10}
 !10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
-!11 = !{!12}
+!11 = !{!12, !17}
 !12 = !DILocalVariable(name: "d", scope: !7, file: !1, line: 2, type: !13)
 !13 = !DIBasicType(name: "double", size: 64, encoding: DW_ATE_float)
 !14 = !DILocation(line: 0, scope: !7)
 !15 = !DILocation(line: 3, column: 2, scope: !7)
 !16 = !DILocation(line: 5, column: 2, scope: !7)
+!17 = !DILocalVariable(name: "c", scope: !7, file: !1, line: 2, type: !18)
+!18 = !DIBasicType(name: "complex", size: 128, encoding: DW_ATE_complex_float)
Index: llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -2469,13 +2469,13 @@
       DwarfExpr.addExpression(std::move(ExprCursor));
       return;
   } else if (Value.isConstantFP()) {
-    if (AP.getDwarfVersion() >= 4 && !AP.getDwarfDebug()->tuneForSCE()) {
+    if (AP.getDwarfVersion() >= 4 && !AP.getDwarfDebug()->tuneForSCE() &&
+        !ExprCursor) {
       DwarfExpr.addConstantFP(Value.getConstantFP()->getValueAPF(), AP);
       return;
-    } else if (Value.getConstantFP()
-                   ->getValueAPF()
-                   .bitcastToAPInt()
-                   .getBitWidth() <= 64 /*bits*/)
+    }
+    if (Value.getConstantFP()->getValueAPF().bitcastToAPInt().getBitWidth() <=
+        64 /*bits*/)
       DwarfExpr.addUnsignedConstant(
           Value.getConstantFP()->getValueAPF().bitcastToAPInt());
     else


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