[project @ 2005-11-08 12:56:04 by simonmar]

gcc's -fstrict-aliasing is biting us when we use the stack to store
different types of objects.  For example:

  *((StgDouble*)((W_)Sp-8)) = *((StgDouble*)((W_)Sp+8));
  Sp[1] = (W_)&s1Cx_info;

gcc feels free to reorder these two lines, because they refer to
differently typed objects, even though the assignment to Sp[1] clearly
aliases the read from the same location.

Trying to fix this by accessing locations using union types might be
possible, but I took the sledgehammer approach of
-fno-strict-aliasing.  This is justified to a certain extent because
our generated C code is derived from a very weakly-typed internal
language (C--).

diff --git a/ghc/compiler/main/DriverPipeline.hs b/ghc/compiler/main/DriverPipeline.hs
index 07fe113..d1fa5ba 100644 (file)
--- a/ghc/compiler/main/DriverPipeline.hs
+++ b/ghc/compiler/main/DriverPipeline.hs
@@ -852,6 +852,23 @@ runPhase cc_phase stop dflags basename suff input_fn get_output_fn maybe_loc
                | otherwise         = As
        output_fn <- get_output_fn next_phase maybe_loc
 
+       let
+         more_hcc_opts =
+#if x86_TARGET_ARCH
+               -- on x86 the floating point regs have greater precision
+               -- than a double, which leads to unpredictable results.
+               -- By default, we turn this off with -ffloat-store unless
+               -- the user specified -fexcess-precision.
+               (if excessPrecision then [] else [ "-ffloat-store" ]) ++
+#endif
+               -- gcc's -fstrict-aliasing allows two accesses to memory
+               -- to be considered non-aliasing if they have different types.
+               -- This interacts badly with the C code we generate, which is
+               -- very weakly typed, being derived from C--.
+               ["-fno-strict-aliasing"]
+
+
+
        SysTools.runCc dflags (
                -- force the C compiler to interpret this file as C when
                -- compiling .hc files, by adding the -x c option.
@@ -868,13 +885,13 @@ runPhase cc_phase stop dflags basename suff input_fn get_output_fn maybe_loc
                       ++ (if hcc && mangle
                             then md_regd_c_flags
                             else [])
+                      ++ (if hcc 
+                            then more_hcc_opts
+                            else [])
                       ++ [ verb, "-S", "-Wimplicit", "-O" ]
                       ++ [ "-D__GLASGOW_HASKELL__="++cProjectVersionInt ]
                       ++ cc_opts
                       ++ split_opt
-#if x86_TARGET_ARCH
-                      ++ (if excessPrecision then [] else [ "-ffloat-store" ])
-#endif
                       ++ include_paths
                       ++ pkg_extra_cc_opts
                       ))