x86: Support BHI_CTRL instruction
Patch adds support for the BHI_CTRL instruction (ibhf) to the x86 disassembler and assembler.
Haochen Jiang submits a patch to add support for the BHI_CTRL instruction, known as ‘ibhf’, to Binutils. This instruction uses REX.W encoding, and importantly, REX2.W is also permissible without causing additional burden. The patch updates the disassembler and assembler to correctly handle this instruction, including adding new test cases for both REX2 and general x86-64 scenarios. The information about this instruction will be available in the Q4 2026 Software Developer’s Manual (SDM).
- proposer
Submitted a patch to add support for the BHI_CTRL instruction, clarifying its encoding and confirming compatibility with REX2.W.
“I just get the clarification that for ibhf, both rex.w and rex2.w took the same effect. Thus, it will not cause extra burden from software side. I have added REX2 test for BHI_CTRL to explicitly show that.”
- reviewer
Provided feedback on the new test cases, specifically requesting the removal of \s whitespace characters in favor of tab characters for better readability and consistency.
“As also said to Rohan on (I think) the AVX2-aux patch, no \s in new tests please. [ ] helps readability, by making the whitesp”
Technical Tradeoffs
- Ensuring correct disassembler output for both REX.W and REX2.W encodings of the BHI_CTRL instruction.
- Integrating new instructions close to release cycles, requiring clear communication about their inclusion.
In Details
This patch introduces support for the BHI_CTRL instruction (opcode ibhf) into the x86 backend of binutils. The instruction, detailed in Intel's May 2025 technical paper on Branch History Injection, uses a specific encoding (F3 48 0F 1E F8) that leverages REX.W. Crucially, REX2.W is also a valid prefix for this instruction, a detail clarified in the patch's rationale. The changes update i386-opc.tbl, i386-gen.c, i386-dis.c, and add new test cases to the gas/testsuite/gas/i386/ directory to cover both the basic and REX2-encoded forms of the instruction, ensuring correct disassembly and…
- BHI_CTRL
- An instruction related to Branch History Injection, likely used for security purposes to mitigate speculative execution attacks.
- ibhf
- The mnemonic for the Branch History Injection instruction.
- REX.W / REX2.W
- Instruction prefixes in x86-64 that extend the instruction set, with the '.W' bit typically signifying 64-bit operand size.
- Disassembler
- A tool that converts machine code into human-readable assembly language.
- SDM
- Software Developer's Manual, a comprehensive guide for programmers detailing processor instructions and features.
- NOP encoding
- No-Operation instruction encoding. Instructions that do nothing but consume CPU cycles. Some NOP encodings can be used by other instructions.