[RFC PATCH, alpha]: ABI change: pass SFmode and SCmode variable arguments by reference

[Uros Bizjak <ubizjak@gmail.com>]

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Hello!

I would like to propose an ABI adjustment for the aloha OSF/1 ABI. To
quote explanation in the patch:

--q--
  /* Pass float and _Complex float variable arguments by reference.
     This avoids 64-bit store from a FP register to a pretend args save area
     and subsequent 32-bit load from the saved location to a FP register.

     Note that 32-bit loads and stores to/from a FP register on alpha reorder
     bits to form a canonical 64-bit value in the FP register.  This fact
     invalidates compiler assumption that 32-bit FP value lives in the lower
     32-bits of the passed 64-bit FP value, so loading the 32-bit value from
     the stored 64-bit location using 32-bit FP load is invalid on alpha.

     This introduces sort of ABI incompatibility, but until _Float32 was
     introduced, C-family languages promoted 32-bit float variable arg to
     a 64-bit double, and it was not allowed to pass float as a varible
     argument.  Passing _Complex float as a variable argument never
     worked on alpha.  Thus, we have no backward compatibility issues
     to worry about, and passing un-promoted _Float32 and _Complex float
     as a variable argument will actually work in the future.  */
--/q--

Another rationale for the adjustment is, that "other" compilers do not
know about _Float32 and _Complex float, the official ABI pre-dates
some of these types by a decade or more (I was not able to find the
authoritative OSF/1 ABI document on the net...), and lastly ... gcc is
the last compiler that keeps this dead architecture alive, so IMO we
can consider it as a de-facto implementer of the ABI.

Following this ABI adjustment, we can also fix libffi, where
libffi.complex/cls_complex_va_float.c says:

--q--
/* Alpha splits _Complex into two arguments.  It's illegal to pass
   float through varargs, so _Complex float goes badly.  In sort of
   gets passed as _Complex double, but the compiler doesn't agree
   with itself on this issue.  */
/* { dg-do run { xfail alpha*-*-* } } */
--/q--

2016-09-02  Uros Bizjak  <ubizjak@gmail.com>

    * config/alpha/alpha.c (alpha_pass_by_reference): Pass un-named
    SFmode and SCmode arguments by reference.

Patch was bootstrapped and regression tested on alphaev68-linux-gnu
for all default languages plus obj-c++ and go.

Any comments?

Uros.

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diff --git a/gcc/config/alpha/alpha.c b/gcc/config/alpha/alpha.c
index 702cd27..81cef4e 100644
--- a/gcc/config/alpha/alpha.c
+++ b/gcc/config/alpha/alpha.c
@@ -5754,8 +5754,29 @@ static bool
 alpha_pass_by_reference (cumulative_args_t ca ATTRIBUTE_UNUSED,
 			 machine_mode mode,
 			 const_tree type ATTRIBUTE_UNUSED,
-			 bool named ATTRIBUTE_UNUSED)
+			 bool named)
 {
+  /* Pass float and _Complex float variable arguments by reference.
+     This avoids 64-bit store from a FP register to a pretend args save area
+     and subsequent 32-bit load from the saved location to a FP register.
+
+     Note that 32-bit loads and stores to/from a FP register on alpha reorder
+     bits to form a canonical 64-bit value in the FP register.  This fact
+     invalidates compiler assumption that 32-bit FP value lives in the lower
+     32-bits of the passed 64-bit FP value, so loading the 32-bit value from
+     the stored 64-bit location using 32-bit FP load is invalid on alpha.
+
+     This introduces sort of ABI incompatibility, but until _Float32 was
+     introduced, C-family languages promoted 32-bit float variable arg to
+     a 64-bit double, and it was not allowed to pass float as a varible
+     argument.  Passing _Complex float as a variable argument never
+     worked on alpha.  Thus, we have no backward compatibility issues
+     to worry about, and passing unpromoted _Float32 and _Complex float
+     as a variable argument will actually work in the future.  */
+
+  if (mode == SFmode || mode == SCmode)
+    return !named;
+
   return mode == TFmode || mode == TCmode;
 }