From: Simon Marlow <marlowsd@gmail.com>
Date: Wed, 19 Nov 2008 16:29:10 +0000 (+0000)
Subject: Fix regTableToCapability() if gcc introduces padding
X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=1d14a6cd5f563507135d450a63cf2bd7e870fe55

Fix regTableToCapability() if gcc introduces padding
Also avoid padding if possible using __attribute__((packed))
Fixes the Windows build
---

diff --git a/rts/Capability.h b/rts/Capability.h
index db9e1e3..8954578 100644
--- a/rts/Capability.h
+++ b/rts/Capability.h
@@ -32,7 +32,11 @@ struct Capability_ {
     // code.  During STG execution, the BaseReg register always points
     // to the StgRegTable of the current Capability (&cap->r).
     StgFunTable f;
-    StgRegTable r;
+    StgRegTable r GNU_ATTRIBUTE(packed);
+       // packed eliminates any padding between f and r.  Not strictly
+       // necessary, but it means the negative offsets for accessing
+       // the fields of f when we are in STG code are as small as
+       // possible.
 
     nat no;  // capability number.
 
@@ -140,10 +144,12 @@ struct Capability_ {
 
 // Converts a *StgRegTable into a *Capability.
 //
+#define OFFSET(s_type, field) ((size_t)&(((s_type*)0)->field))
+
 INLINE_HEADER Capability *
 regTableToCapability (StgRegTable *reg)
 {
-    return (Capability *)((void *)((unsigned char*)reg - sizeof(StgFunTable)));
+    return (Capability *)((void *)((unsigned char*)reg - OFFSET(Capability,r)));
 }
 
 // Initialise the available capabilities.