[all-commits] [llvm/llvm-project] 6bd844: [FuncSpec] Skip SCCP on blocks of dead functions a...

[XChy via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 6bd844812385dd5cb65e08fe9561be9f91ace876
      https://github.com/llvm/llvm-project/commit/6bd844812385dd5cb65e08fe9561be9f91ace876
  Author: XChy <xxs_chy at outlook.com>
  Date:   2025-08-28 (Thu, 28 Aug 2025)

  Changed paths:
    M llvm/include/llvm/Transforms/IPO/FunctionSpecialization.h
    M llvm/lib/Transforms/IPO/FunctionSpecialization.cpp
    M llvm/lib/Transforms/IPO/SCCP.cpp
    A llvm/test/Transforms/FunctionSpecialization/reachable-after-specialization.ll

  Log Message:
  -----------
  [FuncSpec] Skip SCCP on blocks of dead functions and poison their callsites (#154668)

Fixes #153295.
For test case below:
```llvm
define i32 @caller() {
entry:
  %call1 = call i32 @callee(i32 1)
  %call2 = call i32 @callee(i32 0)
  %cond = icmp eq i32 %call2, 0
  br i1 %cond, label %common.ret, label %if.then

common.ret:                                       ; preds = %entry
  ret i32 0

if.then:                                         ; preds = %entry
  %unreachable_call = call i32 @callee(i32 2)
  ret i32 %unreachable_call
}

define internal i32 @callee(i32 %ac) {
entry:
  br label %ai

ai:                                               ; preds = %ai, %entry
  %add = or i32 0, 0
  %cond = icmp eq i32 %ac, 1
  br i1 %cond, label %aj, label %ai

aj:                                               ; preds = %ai
  ret i32 0
}
```
Before specialization, the SCCP solver determines that
`unreachable_call` is unexecutable, as the value of `callee` can only be
zero.
After specializing the call sites `call1` and `call2`, FnSpecializer
announces `callee` is a dead function since all executable call sites
are specialized. However, the unexecutable call sites can become
executable again after solving specialized calls.
In this testcase, `call2` is considered `Overdefined` after
specialization, making `cond` also `Overdefined`. Thus,
`unreachable_call` becomes executable.
This patch skips SCCP on the blocks in dead functions, and poisons the
call sites of dead functions.



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