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I noticed an issue with the RISC-V prologue scanning stack unwinder. We currently read the frame base register (either $sp or $fp) as a signed value. This means that the frame_id's stack_addr field will be a signed value. In other contexts though these registers are data pointers, and so are unsigned. There's not many places where this mismatch actually shows though, but I did find one place. Consider this GDB session: (gdb) maintenance set dwarf unwinders off (gdb) set backtrace past-main on ... (gdb) b main Breakpoint 1 at 0x20400344: file main.c, line 86. (gdb) run ... (gdb) bt #0 main () at main.c:86 #1 0x2040005c in _start () at start.S:59 Backtrace stopped: frame did not save the PC (gdb) info frame 1 Stack frame at 0x80000a1c: pc = 0x2040005c in _start (start.S:59); saved pc = <not saved> Outermost frame: frame did not save the PC caller of frame at 0x80000a1c source language asm. Arglist at 0x80000a1c, args: Locals at 0x80000a1c, Previous frame's sp is 0x80000a1c (gdb) frame address 0x80000a1c No frame at address 0x80000a1c. (gdb) frame address 0xffffffff80000a1c #1 0x2040005c in _start () at start.S:59 59 call main Notice that the 'info frame 1' reports that the frame is at '0x80000a1c', this is the unsigned frame base value, but when I try to select a frame using this address I can't. The reason is that the frame_id for frame #1 actually has the unsigned (and hence sign-extended) stack_addr value. When I use the sign extended address I can correctly select the frame. I propose changing the prologue scanning unwinder to read the frame base as unsigned. After this in the above case I can now do this: (gdb) frame address 0x80000a1c #1 0x2040005c in _start () at start.S:59 59 call main (gdb) frame address 0xffffffff80000a1c No frame at address 0xffffffff80000a1c. Which I think makes more sense. This issue causes failures in gdb.base/frame-selection.exp if you compile for RV32 with a linker script that places the stack in the correct location, which are resolved by this patch. gdb/ChangeLog: * riscv-tdep.c (riscv_frame_cache): Read the frame base register as an unsigned value.
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