[Tcl-bugs] [ tcl-Bugs-2859027 ] Bytecode Compiler & tcl_precision

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Bugs item #2859027, was opened at 2009-09-14 22:15
Message generated for change (Comment added) made by dgp
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Category: 47. Bytecode Compiler
Group: current: 8.5.7
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Rickard Utgren (pixelz)
>Assigned to: Don Porter (dgp)
Summary: Bytecode Compiler & tcl_precision

Initial Comment:
The bytecode compiler breaks on the following snippet:

proc DoPrecision {} { puts "A $::tcl_precision [expr { 1.1 + 2.2 }]"
	if {[catch { incr ::tcl_precision }]} { return }
	puts "B $::tcl_precision [expr { 1.1 + 2.2 }]"
	DoPrecision
}
DoPrecision

Example result:
A 0 3.3000000000000003
B 1 3.0
A 1 3.3000000000000003
B 2 3.0
A 2 3.3000000000000003
B 3 3.0
A 3 3.3000000000000003
B 4 3.0
A 4 3.3000000000000003
B 5 3.0
A 5 3.3000000000000003
B 6 3.0
A 6 3.3000000000000003
B 7 3.0
A 7 3.3000000000000003
B 8 3.0
A 8 3.3000000000000003
B 9 3.0
A 9 3.3000000000000003
B 10 3.0
A 10 3.3000000000000003
B 11 3.0
A 11 3.3000000000000003
B 12 3.0
A 12 3.3000000000000003
B 13 3.0
A 13 3.3000000000000003
B 14 3.0
A 14 3.3000000000000003
B 15 3.0
A 15 3.3000000000000003
B 16 3.0
A 16 3.3000000000000003
B 17 3.0
A 17 3.3000000000000003


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>Comment By: Don Porter (dgp)
Date: 2009-09-15 13:34

Message:

Starting in Tcl 8.5, the expression parser
detects constant expressions such as
" 1.1 + 2.2 " and notices that their value
is completely known at compile time, and
creates bytecode that simply pushes  that
value on the stack.  There is no runtime
computation at all.

This makes perfect sense using the 8.5
defaults where EIAS for double values
actually works.

For legacy support of scripts continuing
to use the ::tcl_precision variable against
advice, the assumption that a constant
expression will always compute the same
result fails.

Perhaps this can be fixed with some additional
detections of changes to the ::tcl_precision
variable triggering a reset of some compile epoch
to force recompile of the expressions?


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