/* -----------------------------------------------------------------------------
- * $Id: TailCalls.h,v 1.5 2000/04/05 14:26:31 panne Exp $
+ * $Id: TailCalls.h,v 1.7 2001/02/14 10:33:05 simonmar Exp $
*
* (c) The GHC Team, 1998-1999
*
stack that GCC hasn't popped yet. Also possibly to fool any
optimisations (a function call often acts as a barrier). Not sure
if any of this is necessary now -- SDM
+
+ Comment to above note: I don't think the __DISCARD__() in JMP_ is
+ necessary. Arguments should be popped from the C stack immediately
+ after returning from a function, as long as we pass -fno-defer-pop
+ to gcc. Moreover, a goto to a first-class label acts as a barrier
+ for optimisations in the same way a function call does.
+ -= chak
*/
/* The goto here seems to cause gcc -O2 to delete all the code after
/* -----------------------------------------------------------------------------
Tail calling on HP
+
+Description of HP's weird procedure linkage, many thanks to Andy Bennet
+<andy_bennett@hp.com>:
+
+I've been digging a little further into the problem of how HP-UX does
+dynamic procedure calls. My solution in the last e-mail inserting an extra
+'if' statement into the JMP_ I think is probably the best general solution I
+can come up with. There are still a few problems with it however: It wont
+work, if JMP_ ever has to call anything in a shared library, if this is
+likely to be required it'll need something more elaborate. It also wont work
+with PA-RISC 2.0 wide mode (64-bit) which uses a different format PLT.
+
+I had some feedback from someone in HP's compiler lab and the problem
+relates to the linker on HP-UX, not gcc as I first suspected. The reason the
+'hsc' executable works is most likely due to a change in 'ld's behaviour for
+performance reasons between your revision and mine.
+
+The major issue relating to this is shared libraries and how they are
+implented under HP-UX. The whole point of the Procedure Label Table (PLT) is
+to allow a function pointer to hold the address of the function and a
+pointer to the library's global data lookup table (DLT) used by position
+independent code (PIC). This makes the PLT absolutely essential for shared
+library calls. HP has two linker introduced assembly functions for dealing
+with dynamic calls, $$dyncall and $$dyncall_external. The former does a
+check to see if the address is a PLT pointer and dereferences if necessary
+or just calls the address otherwise; the latter skips the check and just
+does the indirect jump no matter what.
+
+Since $$dyncall_external runs faster due to its not having the test, the
+linker nowadays prefers to generate calls to that, rather than $$dyncall. It
+makes this decision based on the presence of any shared library. If it even
+smells an sl's existence at link time, it rigs the runtime system to
+generate PLT references for everything on the assumption that the result
+will be slightly more efficient. This is what is crashing GHC since the
+calls it is generating have no understanding of the procedure label proper.
+The only way to get real addresses is to link everything archive, including
+system libraries, at which point it assumes you probably are going to be
+using calls similar to GHC's (its rigged for HP's +ESfic compiler option)
+but uses $$dyncall if necessary to cope, just in case you aren't.
+
-------------------------------------------------------------------------- */
#ifdef hppa1_1_hp_hpux_TARGET
-#define JMP_(cont) \
- do { void *_procedure = (void *)(cont); \
- goto *_procedure; \
+#define JMP_(cont) \
+ do { void *_procedure = (void *)(cont); \
+ if (((int) _procedure) & 2) \
+ _procedure = (void *)(*((int *) (_procedure - 2))); \
+ goto *_procedure; \
} while(0)
#endif /* hppa1_1_hp_hpux_TARGET */
function and these markers is shredded by the mangler.
-------------------------------------------------------------------------- */
+/* The following __DISCARD__() has become necessary with gcc 2.96 on x86.
+ * It prevents gcc from moving stack manipulation code from the function
+ * body (aka the Real Code) into the function prologue, ie, from moving it
+ * over the --- BEGIN --- marker. It should be noted that (like some
+ * other black magic in GHC's code), there is no essential reason why gcc
+ * could not move some stack manipulation code across the __DISCARD__() -
+ * it just doesn't choose to do it at the moment.
+ * -= chak
+ */
#ifndef FB_
-#define FB_ __asm__ volatile ("--- BEGIN ---");
+#define FB_ __asm__ volatile ("--- BEGIN ---"); __DISCARD__ ();
#endif
#ifndef FE_
projects / ghc-hetmet.git / blobdiff