gas: add flex mode to .bundle_lock
Reviewer welcomes a patch adding 'flex' mode to .bundle_lock but requests documentation and justification for oversized sequences.
Abhay Kandpal welcomes Sarvesh Chandra’s first patch to binutils, which adds a ‘flex’ mode to the .bundle_lock directive in the assembler (gas). This mode allows sequences larger than the standard bundle size. The reviewer finds the format acceptable but asks for clarification on why oversized locked sequences are needed and requests documentation updates for as.texi and a gas/NEWS entry.
- reviewer
Welcomes the patch and reviewer, finds the format acceptable, but requests justification for oversized sequences and documentation updates.
“Thanks for the patch, and welcome. The format looks fine to me. Could you say in the commit message what needs oversized locked sequences? gas/doc/as.texi describes a .bundle_lock sequence as staying within the same bundle, which flex changes, so the new argument needs documenting there. A gas/NEWS entry would probably be good too.”
- proposer
Proposes a patch to allow `.bundle_lock` flex to accept sequences larger than the bundle size, keeping existing alignment handling and strict checks for ordinary locks, and adds tests.
“Allow .bundle_lock flex to accept sequences larger than the bundle size. Keep the existing alignment handling and strict checks for ordinary locks. Add tests for straddling, non-straddling, oversized, and nested locks.”
Technical Tradeoffs
- Increased flexibility in instruction sequencing.
- Requires documentation updates and justification for use cases.
In Details
This patch introduces a 'flex' mode to the .bundle_lock pseudo-instruction in the GNU Assembler (gas). This mode relaxes the constraint that sequences locked by .bundle_lock must entirely fit within a single instruction bundle (typically 4 instructions on x86). The change, located in gas/read.c, allows .bundle_lock to accommodate sequences that span across bundle boundaries. The discussion centers on the justification for this flexibility and the necessary documentation updates.
- gas
- The GNU Assembler, part of the GNU Binutils package. It translates assembly language code into machine code.
- .bundle_lock
- An assembler directive used to ensure that a sequence of instructions is kept together within a single instruction bundle, relevant for architectures with instruction bundling.
- instruction bundle
- A group of instructions that can be executed in parallel or as a single unit on certain architectures, like VLIW or some x86 processors.