[PATCH dwarves 6/9] dwarf_loader: Detect optimized parameters with locations having constant values

The following is an example:

0x00899a78:   DW_TAG_subprogram
                DW_AT_low_pc    (0xffffffff879be480)
                DW_AT_high_pc   (0xffffffff879beac4)
                DW_AT_frame_base        (DW_OP_reg7 RSP)
                DW_AT_call_all_calls    (true)
                DW_AT_name      ("split_mem_range")
                DW_AT_decl_file ("/home/yhs/work/bpf-next/arch/x86/mm/init.c")
                DW_AT_decl_line (401)
                DW_AT_prototyped        (true)
                DW_AT_calling_convention        (DW_CC_nocall)
                DW_AT_type      (0x008861cb "int")

0x00899a8c:     DW_TAG_formal_parameter
                  DW_AT_location        (indexed (0x3e) loclist = 0x000b2195:
                     [0xffffffff879be485, 0xffffffff879be49a): DW_OP_reg5 RDI
                     [0xffffffff879be49a, 0xffffffff879beac4): DW_OP_breg7 RSP+0)
                  DW_AT_name    ("mr")
                  DW_AT_decl_file       ("/home/yhs/work/bpf-next/arch/x86/mm/init.c")
                  DW_AT_decl_line       (401)
                  DW_AT_type    (0x00899c88 "map_range *")

0x00899a98:     DW_TAG_formal_parameter
                  DW_AT_location        (indexed (0x41) loclist = 0x000b21d4:
                     [0xffffffff879be480, 0xffffffff879be554): DW_OP_consts +0, DW_OP_stack_value
                     [0xffffffff879be554, 0xffffffff879be56d): DW_OP_consts +1, DW_OP_stack_value
                     [0xffffffff879be56d, 0xffffffff879be572): DW_OP_reg2 RCX
                     [0xffffffff879be572, 0xffffffff879be638): DW_OP_breg7 RSP+8
                     [0xffffffff879be638, 0xffffffff879be63d): DW_OP_reg0 RAX
                     [0xffffffff879be63d, 0xffffffff879be6ef): DW_OP_breg7 RSP+8
                     [0xffffffff879be6ef, 0xffffffff879be6f5): DW_OP_reg14 R14
                     [0xffffffff879be6f5, 0xffffffff879be6fd): DW_OP_breg7 RSP+8
                     [0xffffffff879be6fd, 0xffffffff879be879): DW_OP_reg14 R14
                     [0xffffffff879be879, 0xffffffff879be931): DW_OP_reg12 R12
                     [0xffffffff879be955, 0xffffffff879be961): DW_OP_reg14 R14
                     [0xffffffff879be961, 0xffffffff879be966): DW_OP_reg12 R12
                     [0xffffffff879be966, 0xffffffff879be976): DW_OP_reg14 R14
                     [0xffffffff879be976, 0xffffffff879be9df): DW_OP_reg12 R12
                     [0xffffffff879be9df, 0xffffffff879be9e5): DW_OP_reg14 R14
                     [0xffffffff879be9fc, 0xffffffff879bea24): DW_OP_consts +0, DW_OP_stack_value
                     [0xffffffff879bea24, 0xffffffff879bea74): DW_OP_breg7 RSP+8
                     [0xffffffff879bea74, 0xffffffff879beac4): DW_OP_reg14 R14)
                  DW_AT_name    ("nr_range")
                  DW_AT_decl_file       ("/home/yhs/work/bpf-next/arch/x86/mm/init.c")
                  DW_AT_decl_line       (401)
                  DW_AT_type    (0x008861cb "int")

0x00899aa4:     DW_TAG_formal_parameter
                  DW_AT_location        (indexed (0x3f) loclist = 0x000b21a7:
                     [0xffffffff879be485, 0xffffffff879be4a4): DW_OP_reg4 RSI
                     [0xffffffff879be4a4, 0xffffffff879be4e6): DW_OP_reg12 R12
                     [0xffffffff879be4e6, 0xffffffff879beac4): DW_OP_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value)
                  DW_AT_name    ("start")
                  DW_AT_decl_file       ("/home/yhs/work/bpf-next/arch/x86/mm/init.c")
                  DW_AT_decl_line       (402)
                  DW_AT_type    (0x008861cf "unsigned long")

0x00899ab0:     DW_TAG_formal_parameter
                  DW_AT_location        (indexed (0x40) loclist = 0x000b21c2:
                     [0xffffffff879be485, 0xffffffff879be4a9): DW_OP_reg1 RDX
                     [0xffffffff879be4a9, 0xffffffff879beac4): DW_OP_breg7 RSP+32)
                  DW_AT_name    ("end")
                  DW_AT_decl_file       ("/home/yhs/work/bpf-next/arch/x86/mm/init.c")
                  DW_AT_decl_line       (403)
                  DW_AT_type    (0x008861cf "unsigned long")

The parameter 'nr_range' is a constant and won't consume any ABI register.

Signed-off-by: Yonghong Song <[email protected]>
---
 dwarf_loader.c | 50 ++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 44 insertions(+), 6 deletions(-)

diff --git a/dwarf_loader.c b/dwarf_loader.c
index 684115d..712a957 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -1197,6 +1197,21 @@ struct func_info {
 
 #define	PARM_DEFAULT_FAIL	-1
 #define	PARM_FBREG_FAIL		-2
+#define	PARM_OPTIMIZED_CLANG	-3
+#define	PARM_CONTINUE		-4
+
+static int parameter__multi_exprs(Dwarf_Op *expr, int loc_num) {
+	switch (expr[0].atom) {
+	case DW_OP_lit0 ... DW_OP_lit31:
+	case DW_OP_constu:
+	case DW_OP_consts:
+		if (loc_num != 0)
+			break;
+		return PARM_OPTIMIZED_CLANG;
+	}
+
+	return PARM_CONTINUE;
+}
 
 /* For DW_AT_location 'attr':
  * - if first location is DW_OP_regXX with expected number, return the register;
@@ -1205,7 +1220,7 @@ struct func_info {
  *   list, return the register; otherwise save register for later return
  * - otherwise if no register was found for locations, return PARM_DEFAULT_FAIL.
  */
-static int parameter__reg(Dwarf_Attribute *attr, int expected_reg)
+static int parameter__reg(Dwarf_Attribute *attr, int expected_reg, struct cu *cu, struct conf_load *conf)
 {
 	Dwarf_Addr base, start, end;
 	Dwarf_Op *expr, *entry_ops;
@@ -1229,8 +1244,17 @@ static int parameter__reg(Dwarf_Attribute *attr, int expected_reg)
 		if (exprlen == 2 && expr[exprlen - 1].atom == DW_OP_stack_value)
 			exprlen--;
 
-		if (exprlen != 1)
-			continue;
+		if (exprlen != 1) {
+			if (!cu->producer_clang || !conf->true_signature)
+				continue;
+
+			int res;
+			res = parameter__multi_exprs(expr, loc_num);
+			if (res == PARM_CONTINUE)
+				continue;
+			ret = res;
+			goto out;
+		}
 
 		switch (expr->atom) {
 		/* match DW_OP_regXX at first location */
@@ -1250,6 +1274,16 @@ static int parameter__reg(Dwarf_Attribute *attr, int expected_reg)
 			 */
 			ret = PARM_FBREG_FAIL;
 			break;
+		case DW_OP_lit0 ... DW_OP_lit31:
+		case DW_OP_constu:
+		case DW_OP_consts:
+			if (cu->producer_clang && conf->true_signature) {
+				if (loc_num != 0)
+					break;
+				ret = PARM_OPTIMIZED_CLANG;
+				goto out;
+			}
+			break;
 		/* match DW_OP_entry_value(DW_OP_regXX) at any location */
 		case DW_OP_entry_value:
 		case DW_OP_GNU_entry_value:
@@ -1338,11 +1372,14 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
 
 		if (parm->has_loc) {
 			int expected_reg = cu->register_params[reg_idx];
-			int actual_reg = parameter__reg(&attr, expected_reg);
+			int actual_reg = parameter__reg(&attr, expected_reg, cu, conf);
 
-			if (actual_reg == PARM_DEFAULT_FAIL)
+			if (actual_reg == PARM_DEFAULT_FAIL) {
 				parm->optimized = 1;
-			else if (actual_reg == PARM_FBREG_FAIL || (expected_reg >= 0 && expected_reg != actual_reg))
+			} else if (actual_reg == PARM_OPTIMIZED_CLANG) {
+				parm->optimized = 1;
+				info->skip_idx++;
+			} else if (actual_reg == PARM_FBREG_FAIL || (expected_reg >= 0 && expected_reg != actual_reg)) {
 				/* mark parameters that use an unexpected
 				 * register to hold a parameter; these will
 				 * be problematic for users of BTF as they
@@ -1350,6 +1387,7 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
 				 * contents.
 				 */
 				parm->unexpected_reg = 1;
+			}
 		} else if (!cu->producer_clang && has_const_value) {
 			parm->optimized = 1;
 		} else if (cu->producer_clang) {
-- 
2.47.3