Re: Optimisation and unsafePerformIO
- Original Message -
From: Albert Lai [EMAIL PROTECTED]
To: [EMAIL PROTECTED]
Sent: Wednesday, October 30, 2002 7:35 AM
Subject: Re: Optimisation and unsafePerformIO
David Sabel [EMAIL PROTECTED] writes:
{-# NOINLINE b #-}
b x = if even x then unsafePerformIO getChar else bot
bot = bot
main = do
putChar (b 4)
putChar (b 6)
I am not a compiler implementer (or lawyer, for that matter :)
But I propose this guess. First, both even 4 and even 6 get
constant-folded to True; so b 4 and b 6 both become unsafePerformIO
getChar. Then there is a common subexpression elimination.
No! I used the option -fno-cse, what means that common supexpression
elimination is turned off.
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Re: Optimisation and unsafePerformIO
David Sabel [EMAIL PROTECTED] writes:
{-# NOINLINE b #-}
b x = if even x then unsafePerformIO getChar else bot
bot = bot
main = do
putChar (b 4)
putChar (b 6)
I am not a compiler implementer (or lawyer, for that matter :)
But I propose this guess. First, both even 4 and even 6 get
constant-folded to True; so b 4 and b 6 both become unsafePerformIO
getChar. Then there is a common subexpression elimination.
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RE: Optimisation and unsafePerformIO
Consider the following program:
-
{-# NOINLINE b #-}
b x = if even x then unsafePerformIO getChar else bot
bot = bot
main = do
putChar (b 4)
putChar (b 6)
-
when you compile the programm with the options: -O0
and execute the program you get:
test
ab (That's the input)
ab (That's the ouput)
when you compile the programm with the options: -O1 -fno-cse
you get:
test
ab
aa
You are using unsafePerformIO in an unsafe way. The meaning of your
program depends on whether the compiler implements full laziness or not,
which is a decision left entirely up to the compiler implementor. If
you want to write portable code, don't use unsafePerformIO in this way.
What exactly is it you're trying to achieve? Perhaps we can suggest a
better solution.
Cheers,
Simon
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Re: Optimisation and unsafePerformIO
Of course, I used unsafePerformIO in an unsafe way!
I'm thinking about a way to make unsafePerformIO safe.
Therefore the compiler can't do any transformation the ghc does and I want
to locate these transformations.
- Original Message -
From: Simon Marlow [EMAIL PROTECTED]
To: David Sabel [EMAIL PROTECTED];
[EMAIL PROTECTED]
Sent: Monday, October 28, 2002 10:41 AM
Subject: RE: Optimisation and unsafePerformIO
Consider the following program:
-
{-# NOINLINE b #-}
b x = if even x then unsafePerformIO getChar else bot
bot = bot
main = do
putChar (b 4)
putChar (b 6)
-
when you compile the programm with the options: -O0
and execute the program you get:
test
ab (That's the input)
ab (That's the ouput)
when you compile the programm with the options: -O1 -fno-cse
you get:
test
ab
aa
You are using unsafePerformIO in an unsafe way. The meaning of your
program depends on whether the compiler implements full laziness or not,
which is a decision left entirely up to the compiler implementor. If
you want to write portable code, don't use unsafePerformIO in this way.
What exactly is it you're trying to achieve? Perhaps we can suggest a
better solution.
Cheers,
Simon
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