[Bug debug/109354] New: [arm32] Parameter stored on stack gets wrong debug info with -Og or higher

[Bug debug/109354] New: [arm32] Parameter stored on stack gets wrong debug info with -Og or higher

[sirl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109354

            Bug ID: 109354
           Summary: [arm32] Parameter stored on stack gets wrong debug
                    info with -Og or higher
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sirl at gcc dot gnu.org
  Target Milestone: ---

Hi,

this small testcase

class c1
{
 struct s1
 {
  unsigned int m_n1;
 };

 struct s2
 {
  s1 sM1;
  s1 sM2;
 };

 s2 m_s2;
 int m_bsoM1;
 int m_bsoM2;

 struct OnOffP {
  unsigned int count;
  int so;
  s1 *ts;
 };

 void OnI(struct OnOffP *p);
 void OffI(struct OnOffP *p);

 virtual void OnOff(unsigned int nCount, bool bOff, bool bM1);
};

void c1::OnOff(unsigned int nCount, bool bOff, bool bM1)
{
 struct OnOffP eOffP;
 // 2 variables only added to better show the behaviour
 volatile int xxx = 0xaa00aa;
 volatile int xyz = 0xab00ab;

 eOffP.count = nCount;

 eOffP.so = m_bsoM2;
 eOffP.ts = &(m_s2.sM2);
 if (bM1) {
  eOffP.so = m_bsoM1;
  eOffP.ts = &(m_s2.sM1);
 }

 if (!bOff)
  OnI(&eOffP);
 else
  OffI(&eOffP);

 xxx++;
 xyz++;
}

compiled with "arm-none-eabi-g++-13 -c -mcpu=cortex-r5 -mfpu=vfpv3-d16
-mfloat-abi=hard -fno-exceptions -g -Og" generates wrong debug info for the
nCount parameter.

llvm-dwarfdump-16.0.0 --name nCount --name eOffP -name xyz
arm-debug-info-bug1.o shows:
arm-debug-info-bug1.o:  file format elf32-littlearm

0x000001c5: DW_TAG_formal_parameter
              DW_AT_name        ("nCount")
              DW_AT_decl_file   ("arm-debug-info-bug1.cpp")
              DW_AT_decl_line   (31)
              DW_AT_decl_column (0x1d)
              DW_AT_type        (0x0000002c "unsigned int")
              DW_AT_location    (0x0000005c: 
                 [0x00000000, 0x00000028): DW_OP_reg1 R1
                 [0x00000028, 0x00000054): DW_OP_fbreg -28
                 [0x00000054, 0x00000057): DW_OP_breg1 R1+0
                 [0x00000057, 0x00000078): DW_OP_GNU_entry_value(<decoding
error> f5 01 25 f7 2c f7 00 9f
                 [0x00000078, 0x0000007c): DW_OP_fbreg -28
                 [0x0000007c, 0x0000007f): DW_OP_breg1 R1+0
                 [0x0000007f, 0x00000084): DW_OP_GNU_entry_value(<decoding
error> f5 01 25 f7 2c f7 00 9f)
              DW_AT_GNU_locviews        (0x0000004e)

0x00000201: DW_TAG_variable
              DW_AT_name        ("eOffP")
              DW_AT_decl_file   ("arm-debug-info-bug1.cpp")
              DW_AT_decl_line   (33)
              DW_AT_decl_column (0x10)
              DW_AT_type        (0x0000008b "c1::OnOffP")
              DW_AT_location    (DW_OP_fbreg -20)

0x0000021f: DW_TAG_variable
              DW_AT_name        ("xyz")
              DW_AT_decl_file   ("arm-debug-info-bug1.cpp")
              DW_AT_decl_line   (35)
              DW_AT_decl_column (0x0f)
              DW_AT_type        (0x00000163 "volatile int")
              DW_AT_location    (DW_OP_fbreg -28)

So according to this the debug info says that "nCount" (DW_OP_fbreg -28)
overlaps with "xyz" (DW_OP_fbreg -28), when in fact it overlaps with
"eOffP.count" (DW_OP_fbreg -20).
It's unclear if this is caused by wrong debug info generation, wrong lifetime
or both. When we see cases like this, a common point seems to be that the
offset difference is always 8 (-20 vs -28).
The bug seems to be long-standing though, we saw it with 10.4.0, 12.2.1 and
13.0.1 so far.