Edmund Grimley Evans wrote:
Looking through the tests that fail on aarch64-linux I found ten that depend on an arithmetical operation causing an exception, usually division by zero:tbs/tb0262 test/texception4 test/units/math/tmask test/units/math/tmask2 webtbs/tw3157 webtbs/tw3160a webtbs/tw3160b webtbs/tw3160c webtbs/tw3161 webtbs/tw4100 In AArch64 the instruction for integer division never causes an exception,
The compiler adds an explicit comparison to ensure that in case of an integer division by zero, a run time error is raised anyway. The same is done on PowerPC, which doesn't trigger an exception for integer division by zero either.
and floating-point arithmetic can only cause an exception if an optional part of the architecture is enabled; most hardware doesn't even have the option.
It turns out that this is also optional in the regular ARM architecture, although there most implementations do seem to support it. An AArch64 cpu without the support running plain ARM code won't trigger floating point exceptions either though.
Clearly the compiler could generate code to check for exceptions but it would be very inefficient when those exceptions are not required.
The idea is to add a code generation option for this, so the programmer can decide whether they want the exceptions (and associated performance overhead) or not.
I can't find an explicit statement in the documentation but this example suggests that exceptions from arithmetical operations are not required by the language definition: http://www.freepascal.org/docs-html/ref/refse101.html#x212-22200017.1
While the example does not have the intention of demonstrating that, it is in fact true that according to the ISO Extended Pascal standard nothing special needs to happen in case of a division by zero: a) "A term of the form i div j shall be an error if j is zero" (http://www.pascal-central.com/docs/iso10206.pdf , section 6.8.3.2 — the same goes for "i / j", which is the floating point division). b) definition of Error: "A violation by a program of the requirements of this International Standard that a processor is permitted to leave undetected" (section 3.2)
However, we generally mainly aim for Turbo Pascal and Delphi compatibility, and most code written for those compilers (and for FPC on other platforms) assumes that it is at least possible to configure the platform to raise floating point exceptions.
So, can those tests be disabled on AArch64, by adding
{ %skipcpu=aarch64 }?
I don't think that's the best approach. Here are two possible alternatives:a) we add the "math" unit to the uses clause, check whether floating point exceptions can be enabled (see http://lists.freepascal.org/pipermail/fpc-devel/2015-February/035397.html for the details) and if not, skip the test (that would also cover running the tests when compiled for ARM) b) we leave them as they are until support has been added to the code generator to generate explicit exception checks after each floating point operation that may trigger one (and then specify that this option should be used when compiling those tests; the compiler will ignore it on platforms that don't need it)
My preference goes to option b). I haven't started working on that yet though.
Jonas _______________________________________________ fpc-devel maillist - [email protected] http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
