[PATCH] D138845: [LLVM] Merge -preserve-ll-uselistorder/preserve-bc-uselistorder into -preserve-uselistorder

[Duncan P. N. Exon Smith via Phabricator via llvm-commits]

dexonsmith requested changes to this revision.
dexonsmith added a comment.
This revision now requires changes to proceed.

Separate flags allows vendors (and the default!) to have use-list order on-by-default in bitcode and off-by-default in textual IR.

IMO, the best path to simplification here:

1. Make it illegal or impossible to observe the use-lists of ConstantData (most importantly, ConstantInt and ConstantFloat). E.g., change `replaceAllUsesWith()` to crash for those types and require people to check first.
2. Stop serializing the use-lists of ConstantData (since we know there are no observers). This removes 99% of (a) the space overhead in bitcode and (b) the noise in textual assembly, since we're no longer tracking the use-list of `i1 0`, `i1 1`, `i8 0`, etc.
3. Consider merging the two options and flipping them both on by default since the noise in the textual IR is no longer terrible?
4. Consider removing `-preserve-bc-use-list-order` and just turn it on all the time since it no longer has major overhead?
5. Consider removing `-preserve-ll-use-list-order` and just turn it on all the time since it no longer adds undue noise to the IR?

Note that walking the uses of `i1 0` (or similar) is dangerous since it's operating on all functions/modules/etc. in the LLVMContext. Any code that's doing this should probably be rewritten for other soundness reasons.

I used to have a series of patches to the above effect; there were only a few more test failures to fix before I could land it. Unfortunately I dropped the work since I got busy with something else. Here were some of the fixes I landed:

- r263853 / c3fa1eded258aa38a30911af6602892846efd6d1 (this one was pretty bad)
- r282320 / c82c11428e0ab2ff58a342ca3a615315929a2633
- r282333 / b4798739127cd7748177ac506e6d45cc8cf5e4f8
- r282334 / 4fd9b7e16f98173d730ac1a347f7eae96a9fd2db
- r282337 / b1b208a1f514ccb90b5afba0af49d6aa39dee21b
- r282338 / 11c06ea55a060d5460f716b00ca593e5dd829b17



================
Comment at: llvm/tools/opt/opt.cpp:217
-    cl::desc("Preserve use-list order when writing LLVM bitcode."),
-    cl::init(true), cl::Hidden);
-
----------------
Notice that bitcode defaults to on...


================
Comment at: llvm/tools/opt/opt.cpp:222
-    cl::desc("Preserve use-list order when writing LLVM assembly."),
+static cl::opt<bool> PreserveUseListOrder(
+    "preserve-uselistorder",
+    cl::desc("Preserve use-list order when writing LLVM IR."),
     cl::init(false), cl::Hidden);
 
----------------
... but textual assembly defaults to off.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D138845/new/

https://reviews.llvm.org/D138845