[M3devel] overshifting?

[Jay K]

The front end shouldn't bother asking the backend to shift by overlarge constants, eh?
Granted, it could be from
Shift(a, LAST(INTEGER) + ....);
 
so the frontend can't always know, even if the backend thinks it does.
  I need to try some things there -- code that produces a large shift via overflowing constants.
 
Which..hm..begets the question. I have the backend error for constant overflows. But shouldn't it actually warn? You know, people code these things to trigger the runtime behavior:
 
PROCEDURE Crash()
 BEGIN
   EVAL VAL(2, BOOLEAN);
 END Crash.
  
 
mostly these are warnings at compile, runtime errors.
But some of these things are now barred by m3back, errors, e.g.
 
 EVAL LAST(INTEGER) + 1; 
 
Maybe I do need that warning callback after all?
 
 
 - Jay


________________________________
> From: jay.krell at cornell.edu
> To: hosking at cs.purdue.edu
> Date: Tue, 2 Mar 2010 21:09:54 +0000
> CC: m3devel at elegosoft.com
> Subject: Re: [M3devel] overshifting?
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> I fixed it. It falls back to generating slower code, which is then not hit because
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> it guaranteeably hits a runtime error ahead of it.
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> Perhaps it should just issue a breakpoint there as the code is not reachable.
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> e.g. in C we have:
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> __declspec(noreturn) abort();
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> void F1()
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> abort();
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> /* breakpoint here */
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> Given the guaranteed runtime error, why does the frontend bother asking the backend to do the shift?
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> - Jay
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> ________________________________
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> From: hosking at cs.purdue.edu
> Date: Tue, 2 Mar 2010 14:50:27 -0500
> To: hosking at cs.purdue.edu
> CC: m3devel at elegosoft.com; jay.krell at cornell.edu
> Subject: Re: [M3devel] overshifting?
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> Oh, yes, of course your backend should not blow up on this.
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> On 2 Mar 2010, at 14:46, Tony Hosking wrote:
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> P.S. Here are the signatures of Shift, RightShift, and LeftShift.
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> PROCEDURE Shift (x: T; n: INTEGER): T;
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> For all i such that both i and i - n are in the range [0 .. Word.Size - 1], bit i of the result equals bit i - n of x. The other bits of the result are 0. Thus, shifting by n> 0 is like multiplying by 2^(n)
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> PROCEDURE LeftShift (x: T; n: [0..Size-1]): T;
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> = Shift (x, n)
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> PROCEDURE RightShift (x: T; n: [0..Size-1]): T;
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> = Shift (x, -n)
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> Antony Hosking | Associate Professor | Computer Science | Purdue University
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> 305 N. University Street | West Lafayette | IN 47907 | USA
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> Office +1 765 494 6001 | Mobile +1 765 427 5484
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> On 2 Mar 2010, at 14:44, Tony Hosking wrote:
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> Actually, I should have noticed. Word.LeftShift and Word.RightShift both check the range of the shift parameter. It's not the shift that is out of range. It is the shift constant (in this case 630). If you try Shift(FIRST(LONGINT), -630) you get 0 as expected.
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> So, in fact the run-time error is correct! 630 is not in the range [0..63].
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> On 2 Mar 2010, at 13:54, Tony Hosking wrote:
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> I'm trying to understand this. Surely shifting right will never be out of range? Looks like something is broken.
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> On 2 Mar 2010, at 09:04, Jay K wrote:
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> PutLH(Long.RightShift(FIRST(LONGINT), 630)); PutT("\n");
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> I'm guessing the warning is all you're supposed to get here?
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> C:\dev2\cm3.2\m3-sys\m3tests\src\p2\p227>cm3
> --- building in NT386 ---
> new source -> compiling Main.m3
> "..\Main.m3", line 894: warning: value out of range
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> ***
> *** runtime error:
> *** An enumeration or subrange value was out of range.
> *** file "..\src\TWordN.m3", line 129
> ***
> Stack trace:
> FP PC Procedure
> --------- --------- -------------------------------
> 0x12f0dc 0x48791a RightShift + 0x35 in ..\src\TWordN.m3
> 0x12f15c 0x47c62f doshift + 0x2c6 in ..\src\Stackx86.m3
> 0x12f188 0x4485d2 shift_right + 0x1b2 in ..\src\M3x86.m3
> 0x12f1ac 0x6414ed shift_right + 0xf5 in ..\src\M3CG_Check.m3
> 0x12f1d4 0x505a4f Shift_right + 0x87 in ..\src\misc\CG.m3
> 0x12f1f8 0x570641 CompileR + 0x1da in ..\NT386\LongShift.m3 => ..\src\builti
> nWord\Shift.mg
> 0x12f218 0x5cec2f Compile + 0x83 in ..\src\exprs\CallExpr.m3
> 0x12f234 0x5bfee4 Compile + 0x53 in ..\src\exprs\Expr.m3
> 0x12f274 0x5d19a7 EmitChecks + 0xdf in ..\src\exprs\CheckExpr.m3
> 0x12f2b4 0x5c8b15 GenOrdinal + 0xa6 in ..\src\values\Formal.m3
> ......... ......... ... more frames ...
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