[M3devel] overshifting?

[Jay K]

I fixed it. It falls back to generating slower code, which is then not hit because

it guaranteeably hits a runtime error ahead of it.

Perhaps it should just issue a breakpoint there as the code is not reachable.

e.g. in C we have:

 

__declspec(noreturn) abort();

 

void F1()

{
  abort();

 /* breakpoint here */

}

 

Given the guaranteed runtime error, why does the frontend bother asking the backend to do the shift?

 

 - Jay
 


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?


Oh, yes, of course your backend should not blow up on this.





On 2 Mar 2010, at 14:46, Tony Hosking wrote:

P.S.  Here are the signatures of Shift, RightShift, and LeftShift.


PROCEDURE Shift (x: T; n: INTEGER): T;

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)PROCEDURE LeftShift (x: T; n: [0..Size-1]): T;

= Shift (x, n)PROCEDURE RightShift (x: T; n: [0..Size-1]): T;

= Shift (x, -n)




Antony Hosking | Associate Professor | Computer Science | Purdue University
305 N. University Street | West Lafayette | IN 47907 | USA
Office +1 765 494 6001 | Mobile +1 765 427 5484



On 2 Mar 2010, at 14:44, Tony Hosking wrote:



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.
So, in fact the run-time error is correct!  630 is not in the range [0..63].


On 2 Mar 2010, at 13:54, Tony Hosking wrote:


I'm trying to understand this.  Surely shifting right will never be out of range?  Looks like something is broken.


On 2 Mar 2010, at 09:04, Jay K wrote:

PutLH(Long.RightShift(FIRST(LONGINT), 630)); PutT("\n");
 
 
I'm guessing the warning is all you're supposed to get here?
 
 
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

***
*** 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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