From b45d18f19ec9507d7561c4d503a5a189214f3f77 Mon Sep 17 00:00:00 2001 From: Hannes Domani Date: Sat, 16 Dec 2023 11:24:16 +0100 Subject: Use function entry point record only for entry values PR28987 notes that optimized code sometimes shows the wrong value of variables at the entry point of a function, if some code was optimized away and the variable has multiple values stored in the debug info for this location. In this example: ``` void foo() { int l_3 = 5, i = 0; for (; i < 8; i++) ; test(l_3, i); } ``` When compiled with optimization, the entry point of foo is at the test() function call, since everything else is optimized away. The debug info of i looks like this: ``` (gdb) info address i Symbol "i" is multi-location: Base address 0x140001600 Range 0x13fd41600-0x13fd41600: the constant 0 Range 0x13fd41600-0x13fd41600: the constant 1 Range 0x13fd41600-0x13fd41600: the constant 2 Range 0x13fd41600-0x13fd41600: the constant 3 Range 0x13fd41600-0x13fd41600: the constant 4 Range 0x13fd41600-0x13fd41600: the constant 5 Range 0x13fd41600-0x13fd41600: the constant 6 Range 0x13fd41600-0x13fd41600: the constant 7 Range 0x13fd41600-0x13fd4160f: the constant 8 (gdb) p i $1 = 0 ``` Currently, when at the entry point of a function, it will always show the initial value (here 0), while the user would expect the last value (here 8). This logic was introduced for showing the entry-values of function arguments if they are available, but for some reason this was added for non-entry-values as well. One of the tests of amd64-entry-value.exp shows the same problem for function arguments, if you "break stacktest" in the following example, you stop at this line: ``` 124 static void __attribute__((noinline, noclone)) 125 stacktest (int r1, int r2, int r3, int r4, int r5, int r6, int s1, int s2, 126 double d1, double d2, double d3, double d4, double d5, double d6, 127 double d7, double d8, double d9, double da) 128 { 129 s1 = 3; 130 s2 = 4; 131 d9 = 3.5; 132 da = 4.5; 133 -> e (v, v); 134 asm ("breakhere_stacktest:"); 135 e (v, v); 136 } ``` But `bt` still shows the entry values: ``` s1=s1@entry=11, s2=s2@entry=12, ..., d9=d9@entry=11.5, da=da@entry=12.5 ``` I've fixed this by only using the initial values when explicitely looking for entry values. Now the local variable of the first example is as expected: ``` (gdb) p i $1 = 8 ``` And the test of amd64-entry-value.exp shows the expected current and entry values of the function arguments: ``` s1=3, s1@entry=11, s2=4, s2@entry=12, ..., d9=3.5, d9@entry=11.5, da=4.5, da@entry=12.5 ``` Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28987 Tested-By: Guinevere Larsen Approved-By: Tom Tromey --- gdb/dwarf2/loc.c | 7 ++++--- gdb/dwarf2/loc.h | 3 ++- gdb/testsuite/gdb.arch/amd64-entry-value.exp | 2 +- 3 files changed, 7 insertions(+), 5 deletions(-) (limited to 'gdb') diff --git a/gdb/dwarf2/loc.c b/gdb/dwarf2/loc.c index 5b2d58a..c15221e 100644 --- a/gdb/dwarf2/loc.c +++ b/gdb/dwarf2/loc.c @@ -363,7 +363,8 @@ decode_debug_loc_dwo_addresses (dwarf2_per_cu_data *per_cu, const gdb_byte * dwarf2_find_location_expression (const dwarf2_loclist_baton *baton, - size_t *locexpr_length, const CORE_ADDR pc) + size_t *locexpr_length, const CORE_ADDR pc, + bool at_entry) { dwarf2_per_objfile *per_objfile = baton->per_objfile; struct objfile *objfile = per_objfile->objfile; @@ -456,7 +457,7 @@ dwarf2_find_location_expression (const dwarf2_loclist_baton *baton, loc_ptr += bytes_read; } - if (low == high && unrel_pc == low) + if (low == high && unrel_pc == low && at_entry) { /* This is entry PC record present only at entry point of a function. Verify it is really the function entry point. */ @@ -3920,7 +3921,7 @@ loclist_read_variable_at_entry (struct symbol *symbol, frame_info_ptr frame) if (frame == NULL || !get_frame_func_if_available (frame, &pc)) return value::allocate_optimized_out (symbol->type ()); - data = dwarf2_find_location_expression (dlbaton, &size, pc); + data = dwarf2_find_location_expression (dlbaton, &size, pc, true); if (data == NULL) return value::allocate_optimized_out (symbol->type ()); diff --git a/gdb/dwarf2/loc.h b/gdb/dwarf2/loc.h index 5cf824d..94e1fbe 100644 --- a/gdb/dwarf2/loc.h +++ b/gdb/dwarf2/loc.h @@ -39,7 +39,8 @@ extern unsigned int entry_values_debug; const gdb_byte *dwarf2_find_location_expression (const dwarf2_loclist_baton *baton, size_t *locexpr_length, - CORE_ADDR pc); + CORE_ADDR pc, + bool at_entry = false); /* Find the frame base information for FRAMEFUNC at PC. START is an out parameter which is set to point to the DWARF expression to diff --git a/gdb/testsuite/gdb.arch/amd64-entry-value.exp b/gdb/testsuite/gdb.arch/amd64-entry-value.exp index 3c666ac..c7fea22 100644 --- a/gdb/testsuite/gdb.arch/amd64-entry-value.exp +++ b/gdb/testsuite/gdb.arch/amd64-entry-value.exp @@ -77,7 +77,7 @@ gdb_continue_to_breakpoint "entry_stack: stacktest" gdb_test "bt" \ [multi_line \ - "^#0 +stacktest *\\(r1=r1@entry=1, r2=r2@entry=2, \[^\r\n\]+, s1=s1@entry=11, s2=s2@entry=12, \[^\r\n\]+, d9=d9@entry=11\\.5, da=da@entry=12\\.5\\) \[^\r\n\]*" \ + "^#0 +stacktest *\\(r1=r1@entry=1, r2=r2@entry=2, \[^\r\n\]+, s1=3, s1@entry=11, s2=4, s2@entry=12, \[^\r\n\]+, d9=3\\.5, d9@entry=11\\.5, da=4\\.5, da@entry=12\\.5\\) \[^\r\n\]*" \ "#1 +0x\[0-9a-f\]+ in main .*"] \ "entry_stack: bt at entry" -- cgit v1.1