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LibJS: Drop dead entry GetLexicalEnvironment loads

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Teach the Rust bytecode generator to treat the synthetic entry
GetLexicalEnvironment as a removable prologue load.

We still model reg4 as the saved entry lexical environment during
codegen, but assemble() now deletes that load when no emitted
instruction refers to the saved environment register. This keeps the
semantics of unwinding and environment restoration intact while letting
empty functions and other simple bodies start at their first real
instruction.
This commit is contained in:
Andreas Kling
2026-04-13 13:16:17 +02:00
committed by Andreas Kling
parent 88d4d1b1a6
commit 3a08f7b95f
Notes: github-actions[bot] 2026-04-13 16:31:04 +00:00 
Author: https://github.com/awesomekling
Commit: https://github.com/LadybirdBrowser/ladybird/commit/3a08f7b95f8
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/8891
Reviewed-by: https://github.com/shannonbooth
135 changed files with 1845 additions and 2126 deletions
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25
Tests/LibJS/Bytecode/expected/eval-same-function.txt
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@@ -5,10 +5,9 @@ $e6445d4a eval-same-function.js:10:1
[0] = Undefined
block0:
[ 0] GetLexicalEnvironment dst:reg4
[ 8] GetGlobal dst:reg6, `foo`
[ 20] Call dst:reg5, callee:reg6, this_value:Undefined, foo
[ 40] End value:reg5
[ 0] GetGlobal dst:reg6, `foo`
[ 18] Call dst:reg5, callee:reg6, this_value:Undefined, foo
[ 38] End value:reg5
foo$a4ff2c4f eval-same-function.js:6:9
@@ -20,13 +19,12 @@ foo$a4ff2c4f eval-same-function.js:6:9
[2] = Int32(42)
block0:
[ 0] GetLexicalEnvironment dst:reg4
[ 8] CreateArguments is_immutable:false
[ 18] GetCalleeAndThisFromEnvironment callee:reg6, this_value:reg7, `eval`
[ 30] CallDirectEval dst:reg5, callee:reg6, this_value:reg7, eval, arguments:[String("var x = 1")]
[ 58] GetBinding dst:reg6, `Number`
[ 70] CallConstruct dst:reg5, callee:reg6, Number, arguments:[Int32(42)]
[ 90] Return value:reg5
[ 0] CreateArguments is_immutable:false
[ 10] GetCalleeAndThisFromEnvironment callee:reg6, this_value:reg7, `eval`
[ 28] CallDirectEval dst:reg5, callee:reg6, this_value:reg7, eval, arguments:[String("var x = 1")]
[ 50] GetBinding dst:reg6, `Number`
[ 68] CallConstruct dst:reg5, callee:reg6, Number, arguments:[Int32(42)]
[ 88] Return value:reg5
eval$3c61584c line 1, column 1
@@ -37,6 +35,5 @@ eval$3c61584c line 1, column 1
[1] = Undefined
block0:
[ 0] GetLexicalEnvironment dst:reg4
[ 8] SetLexicalBinding `x`, src:Int32(1)
[ 20] End value:Undefined
[ 0] SetLexicalBinding `x`, src:Int32(1)
[ 18] End value:Undefined