JIT: fix DWARF register numbers and encoding for APX eGPRs

[DeepakRajendrakumaran]

Three related fixes to the Unix CFI unwind path for APX.

1. unwindPush2Pop2CFI described PUSH2 as two separate pushes

. PUSH2 moves RSP by 2 * REGSIZE_BYTES in one step, so it needs a single CFA adjustment of the full amount. Emit one CFI_ADJUST_CFA_OFFSET plus a CFI_REL_OFFSET per callee-saved register, with reg1 at REGSIZE_BYTES and reg2 at 0, matching Intel's PUSH2 semantics ([rsp] = reg2, [rsp + 8] = reg1). This matches what LLVM emits: https://godbolt.org/z/z179oGeq5.

Details:
unwindPush2Pop2CFI was a placeholder that called unwindPushPopCFI twice.
That is correct but describes a single instruction as two, and it left two
adjacent latent bugs on the eGPR path unexercised. Three changes:

unwind.cpp — emit one DW_CFA_def_cfa_offset of 2 * REGSIZE_BYTES
per push2 instead of two of one slot each, matching LLVM's
X86FrameLowering. Intel PUSH2 reg1, reg2 stores [rsp]=reg2 and
[rsp+8]=reg1, so reg1 takes the higher slot: CFI_REL_OFFSET gets
REGSIZE_BYTES for reg1 and 0 for reg2.

How the CFI changed

Measured with unwindTest, a NativeAOT linux-x64 app built with
--codegenopt:EnableAPX=1 + EnableApxPP2=1 + EnableApxPPHint=1, whose frames
exhaust the callee-saved integer set so the JIT pairs the spills. Built and run
both ways on APX hardware.

For a prolog containing push2p %r14,%r15 (AT&T; Intel PUSH2 r15, r14):

  DW_CFA_advance_loc: 6
- DW_CFA_def_cfa_offset: +24
+ DW_CFA_def_cfa_offset: +32
  DW_CFA_offset: R15 -24
- DW_CFA_def_cfa_offset: +32
  DW_CFA_offset: R14 -32

2. mapRegNumToDwarfReg assigned r16-r31 the numbers 16-31.

The x86-64 psABI assigns them 130-145; 16 is the return-address column and 17-32 are XMM0-XMM15. Emitting 16-31 would alias r16 onto the return address and r17-r31 onto XMM0-XMM14: https://lkml.rescloud.iu.edu/hypermail/linux/kernel/2605.3/09638.html.

3. DwarfFde.cs wrote the register operand of DW_CFA_def_cfa_register and DW_CFA_def_cfa as a raw byte.

That is a valid ULEB128 encoding only for values <= 127. 130 encodes as 0x82 0x01; a bare 0x82 sets the continuation bit, swallowing the next byte and desynchronising the rest of the CFI program. Use DwarfHelper.WriteULEB128, as the CFI_REL_OFFSET path already does.

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🔵 Needs a closer look

Unresolved findings require debug-register mapping updates, regression tests, and a comment correction.

Pull request overview

Fixes Unix DWARF/CFI generation for APX eGPRs and PUSH2 prolog instructions.

Changes:

• Corrects APX eGPR mappings to DWARF registers 130–145.
• Encodes CFA register operands as ULEB128.
• Emits accurate PUSH2 CFA adjustments and spill offsets.

File summaries

File	Summary
src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/ObjectWriter/Dwarf/DwarfFde.cs	Corrects ULEB128 encoding; automated regression coverage requested.
src/coreclr/jit/unwindamd64.cpp	Corrects APX eGPR mappings; debug-info register mapping also requires alignment.
src/coreclr/jit/unwind.cpp	Emits accurate PUSH2 CFI; automated APX regression coverage requested.
src/coreclr/inc/cfi.h	Documents DWARF register ranges; register-range comment needs correction.

Review details

Suppressed comments (4)

src/coreclr/inc/cfi.h:23

• The register range in this comment is inaccurate: DWARF register 16 is the return-address (RIP) column, not a general-purpose register. Please document 0-15 as the general-purpose registers and 16 separately so the comment does not imply that R16 uses DWARF register 16.

    short DwarfReg;          // Dwarf register number. For x64: 0-16 general purpose and the
                             // return-address column, 17-32 XMM0-XMM15, and 130-145 for the
                             // APX eGPRs r16-r31.

src/coreclr/jit/unwind.cpp:201

• Could this add an automated regression test for the Unix APX CFI path? The existing unwind tests do not enable APX/PP2 or validate the emitted .eh_frame, so they would not catch either the single PUSH2 CFA adjustment/slot ordering or a future regression in encoding register numbers 130–145 as ULEB128. The manual APX run described for this change is useful, but without a repeatable test these fixes can silently regress.

    createCfiCode(func, cbProlog, CFI_ADJUST_CFA_OFFSET, DWARF_REG_ILLEGAL, 2 * REGSIZE_BYTES);

src/coreclr/jit/unwindamd64.cpp:76

• This APX mapping only fixes the CFI producer. The NativeAOT DWARF variable-location path still passes register numbers 16-31 through DwarfExpressionBuilder.DwarfRegNum, where those values are defined as XMM0-XMM15 and encoded as DWARF 17-32. An APX method with debug variable info would therefore identify an R16-R31 value as the wrong register. Please update that mapping as part of this change, or explicitly scope the fix to .eh_frame and track the debug-info path separately.

        // The x86-64 psABI assigns the APX eGPRs 130-145.
        case REG_R16:
            dwarfReg = 130;

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/ObjectWriter/Dwarf/DwarfFde.cs:85

• Please add an automated regression for the new extended-register encoding and PUSH2 CFI shape. The existing APX scenario exercises the current low-register callee-save set, so it does not produce a 130-145 operand and would not catch the original one-byte encoding desynchronizing the following CFI instructions; it also does not assert the single CFA advance and two slot offsets. A focused synthetic CFI byte-level test or an APX unwind test should cover these cases.

                    case CFI_OPCODE.CFI_DEF_CFA_REGISTER:
                        cfiCode[cfiCodeOffset++] = DW_CFA_def_cfa_register;
                        // The register operand is ULEB128, not a raw byte. Identical output for
                        // registers <= 127, but the APX eGPRs are 130-145.
                        cfiCodeOffset += DwarfHelper.WriteULEB128(cfiCode.AsSpan(cfiCodeOffset), (uint)dwarfReg);

• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Lite