http://d.puremagic.com/issues/show_bug.cgi?id=4169

           Summary: building dmd with a modern gcc produces a buggy
                    compiler
           Product: D
           Version: unspecified
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: braddr@puremagic.com


--- Comment #0 from Brad Roberts <braddr@puremagic.com> 2010-05-09 19:19:52 PDT ---
modern versions of DMD build with strict alias rules.  Somewhere in DMD the rules are violated sufficiently to produce a compiler that doesn't work correctly enough to past the test suite.  A mostly (but possibly not completely) reduced test case:

----
import std.math;

void foo()
{
    float f[1];
    for (int i = 0; i < f.length; i++)
        assert(isnan(f[i]));
}


int main()
{
    foo();
    return 0;
}
----

With dmd as shipped, the test passes.  With dmd build with g++ 4.4, it fails. Adding -fno-strict-aliasing to the compilation flags for dmd produces a compiler that works (at least well enough to pass the above test).

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugzilla@digitalmars.com


--- Comment #1 from Walter Bright <bugzilla@digitalmars.com> 2010-05-09 19:39:10 PDT ---
Wikipedia says: "To enable such optimizations in a predictable manner, the ISO standard for the C programming language (including its newer C99 edition) specifies that it is illegal (with some exceptions) for pointers of different types to reference the same memory location. This rule, known as "strict aliasing", allows impressive increases in performance[citation needed], but has been known to break some otherwise valid code. Several software projects intentionally violate this portion of the C99 standard. For example, Python 2.x did so to implement reference counting,[1] and required changes to the basic object structs in Python 3 to enable this optimisation. The Linux kernel does this because strict aliasing causes problems with optimization of inlined code.[2] In such cases, when compiled with gcc, the option -fno-strict-aliasing is invoked to prevent unwanted or invalid optimizations that could produce incorrect code."

The compiler does do a lot of aliasing, so I don't know if this is a gcc bug or a dmd bug. The way to track it down is to compile half of dmd with no-strict-aliasing, the other half without, see if it still fails/succeeds. This isolates it down to which half has the problem. Rinse, repeat, until the offending source file is isolated.

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--- Comment #2 from Brad Roberts <braddr@puremagic.com> 2010-05-09 21:24:44 PDT ---
One better, for whomever gets to this one first, add -Wstrict-aliasing and check to see if the reported problems are sufficient to fix it.  gcc won't report _every_ violation, but it does report some meaningful subset of them.

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--- Comment #3 from Brad Roberts <braddr@puremagic.com> 2010-05-15 19:02:15 PDT ---
Created an attachment (id=631)
fix all aliasing warnings found by gcc 4.4.3

This change sets fixes all the aliasing warnings found by gcc 4.4.3 with -Wstrict-aliasing.  The dmd test suite passes now, where is it didn't before unless built with -fno-strict-aliasing.

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


--- Comment #4 from Walter Bright <bugzilla@digitalmars.com> 2010-05-25 15:09:04 PDT ---
http://www.dsource.org/projects/dmd/changeset/501

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Brad Roberts <braddr@puremagic.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |


--- Comment #5 from Brad Roberts <braddr@puremagic.com> 2010-05-25 20:55:50 PDT ---
Looks like you missed one file's changes:

diff --git a/src/mtype.c b/src/mtype.c
--- a/src/mtype.c
+++ b/src/mtype.c
@@ -2832,14 +2832,17 @@ Expression *TypeBasic::defaultInit(Loc loc)
      * so that uninitialised variables can be
      * detected even if exceptions are disabled.
      */
-    unsigned short snan[8] = { 0, 0, 0, 0xA000, 0x7FFF };
+    union {
+        unsigned short us[8];
+        long double    ld;
+    } snan = {{ 0, 0, 0, 0xA000, 0x7FFF }};
     /*
      * Although long doubles are 10 bytes long, some
      * C ABIs pad them out to 12 or even 16 bytes, so
      * leave enough space in the snan array.
      */
     assert(REALSIZE <= sizeof(snan));
-    d_float80 fvalue = *(long double*)snan;
+    d_float80 fvalue = snan.ld;
 #endif

 #if LOGDEFAULTINIT


The rest of the compiler builds w/o aliasing warnings on my box from tip of svn now.

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|                            |FIXED


--- Comment #6 from Walter Bright <bugzilla@digitalmars.com> 2010-05-26 12:47:48 PDT ---
http://www.dsource.org/projects/dmd/changeset/502

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