diff --git a/libitm/beginend.cc b/libitm/beginend.cc
index a3bf549..bd7b19e 100644
--- a/libitm/beginend.cc
+++ b/libitm/beginend.cc
@@ -165,7 +165,7 @@ GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb)
   if (unlikely(prop & pr_undoLogCode))
     GTM_fatal("pr_undoLogCode not supported");
 
-#if defined(USE_HTM_FASTPATH) && !defined(HTM_CUSTOM_FASTPATH)
+#ifdef USE_HTM_FASTPATH
   // HTM fastpath.  Only chosen in the absence of transaction_cancel to allow
   // using an uninstrumented code path.
   // The fastpath is enabled only by dispatch_htm's method group, which uses
@@ -187,6 +187,7 @@ GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb)
   // indeed in serial mode, and HW transactions should never need serial mode
   // for any internal changes (e.g., they never abort visibly to the STM code
   // and thus do not trigger the standard retry handling).
+#ifndef HTM_CUSTOM_FASTPATH
   if (likely(htm_fastpath && (prop & pr_hasNoAbort)))
     {
       for (uint32_t t = htm_fastpath; t; t--)
@@ -237,6 +238,49 @@ GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb)
 	    }
 	}
     }
+#else
+  // If we have a custom HTM fastpath in ITM_beginTransaction, we implement
+  // just the retry policy here.  We communicate with the custom fastpath
+  // through additional property bits and return codes, and either transfer
+  // control back to the custom fastpath or run the fallback mechanism.  The
+  // fastpath synchronization algorithm itself is the same.
+  // pr_HTMRetryableAbort states that a HW transaction started by the custom
+  // HTM fastpath aborted, and that we thus have to decide whether to retry
+  // the fastpath (returning a_tryHTMFastPath) or just proceed with the
+  // fallback method.
+  if (likely(htm_fastpath && (prop & pr_HTMRetryableAbort)))
+    {
+      tx = gtm_thr();
+      if (unlikely(tx == NULL))
+        {
+          // See below.
+          tx = new gtm_thread();
+          set_gtm_thr(tx);
+        }
+      // If this is the first abort, reset the retry count.  We abuse
+      // restart_total for the retry count, which is fine because our only
+      // other fallback will use serial transactions, which don't use
+      // restart_total but will reset it when committing.
+      if (!(prop & pr_HTMRetriedAfterAbort))
+	tx->restart_total = htm_fastpath;
+
+      if (--tx->restart_total > 0)
+	{
+	  // Wait until any concurrent serial-mode transactions have finished.
+	  // Essentially the same code as above.
+	  if (serial_lock.is_write_locked())
+	    {
+	      if (tx->nesting > 0)
+		goto stop_custom_htm_fastpath;
+	      serial_lock.read_lock(tx);
+	      serial_lock.read_unlock(tx);
+	    }
+	  // Let ITM_beginTransaction retry the custom HTM fastpath.
+	  return a_tryHTMFastPath;
+	}
+    }
+ stop_custom_htm_fastpath:
+#endif
 #endif
 
   tx = gtm_thr();
diff --git a/libitm/config/linux/rwlock.h b/libitm/config/linux/rwlock.h
index 428299f..c761edf 100644
--- a/libitm/config/linux/rwlock.h
+++ b/libitm/config/linux/rwlock.h
@@ -39,6 +39,11 @@ struct gtm_thread;
 //
 // In this implementation, writers are given highest priority access but
 // read-to-write upgrades do not have a higher priority than writers.
+//
+// Do not change the layout of this class; it must remain a POD type with
+// standard layout, and the WRITERS field must be first (i.e., so the
+// assembler code can assume that its address is equal to the address of the
+// respective instance of the class).
 
 class gtm_rwlock
 {
diff --git a/libitm/config/posix/rwlock.h b/libitm/config/posix/rwlock.h
index 2e41528..b2fd517 100644
--- a/libitm/config/posix/rwlock.h
+++ b/libitm/config/posix/rwlock.h
@@ -44,19 +44,25 @@ struct gtm_thread;
 //
 // In this implementation, writers are given highest priority access but
 // read-to-write upgrades do not have a higher priority than writers.
+//
+// Do not change the layout of this class; it must remain a POD type with
+// standard layout, and the SUMMARY field must be first (i.e., so the
+// assembler code can assume that its address is equal to the address of the
+// respective instance of the class).
 
 class gtm_rwlock
 {
-  pthread_mutex_t mutex;	        // Held if manipulating any field.
-  pthread_cond_t c_readers;	        // Readers wait here
-  pthread_cond_t c_writers;	        // Writers wait here for writers
-  pthread_cond_t c_confirmed_writers;	// Writers wait here for readers
-
   static const unsigned a_writer  = 1;	// An active writer.
   static const unsigned w_writer  = 2;	// The w_writers field != 0
   static const unsigned w_reader  = 4;  // The w_readers field != 0
 
   std::atomic<unsigned int> summary;	// Bitmask of the above.
+
+  pthread_mutex_t mutex;	        // Held if manipulating any field.
+  pthread_cond_t c_readers;	        // Readers wait here
+  pthread_cond_t c_writers;	        // Writers wait here for writers
+  pthread_cond_t c_confirmed_writers;	// Writers wait here for readers
+
   unsigned int a_readers;	// Nr active readers as observed by a writer
   unsigned int w_readers;	// Nr waiting readers
   unsigned int w_writers;	// Nr waiting writers
diff --git a/libitm/config/x86/sjlj.S b/libitm/config/x86/sjlj.S
index 993f698..3c41537 100644
--- a/libitm/config/x86/sjlj.S
+++ b/libitm/config/x86/sjlj.S
@@ -24,6 +24,7 @@
 
 
 #include "asmcfi.h"
+#include "config.h"
 
 #define CONCAT1(a, b) CONCAT2(a, b)
 #define CONCAT2(a, b) a ## b
@@ -52,6 +53,19 @@
 #  endif
 #endif
 
+/* These are duplicates of the canonical definitions in libitm.h.  Note that
+   the code relies on pr_uninstrumentedCode == a_runUninstrumentedCode.  */
+#define pr_uninstrumentedCode	0x02
+#define pr_hasNoAbort		0x08
+#define pr_HTMRetryableAbort	0x800000
+#define pr_HTMRetriedAfterAbort	0x1000000
+#define a_runInstrumentedCode	0x01
+#define a_runUninstrumentedCode	0x02
+#define a_tryHTMFastPath	0x20
+
+#define _XABORT_EXPLICIT	(1 << 0)
+#define _XABORT_RETRY		(1 << 1)
+
 	.text
 
 	.align 4
@@ -60,20 +74,80 @@
 SYM(_ITM_beginTransaction):
 	cfi_startproc
 #ifdef __x86_64__
+#ifdef HAVE_AS_RTM
+	/* Custom HTM fast path.  We start the HW transaction here and let
+	   gtm_thread::begin_transaction (aka GTM_begin_transaction) decide
+	   how to proceed on aborts: We either retry the fast path, or fall
+	   back to another execution method.  RTM restores all registers after
+	   a HW transaction abort, so we can do the SW setjmp after aborts,
+	   and we have to because we might choose a SW fall back.  However,
+	   we have to explicitly save/restore the first argument (edi).  */
+	cmpl	$0, SYM(gtm_htm_fastpath)(%rip)
+	jz	.Lno_htm
+	testl	$pr_hasNoAbort, %edi
+	jz	.Lno_htm
+.Lhtm_fastpath:
+	xbegin	.Ltxn_abort
+	/* Monitor the serial lock (specifically, the 32b writer/summary field
+	   at its start), and only continue if there is no serial-mode
+	   transaction.  Note that we might be just a nested transaction and
+	   our outermost transaction might be in serial mode; we check for
+	   this case in the retry policy implementation.  */
+	cmpl	$0, SYM(gtm_serial_lock)(%rip)
+	jnz	1f
+	/* Everything is good.  Run the transaction, preferably using the
+	   uninstrumented code path.  Note that the following works because
+	   pr_uninstrumentedCode == a_runUninstrumentedCode.  */
+	andl	$pr_uninstrumentedCode, %edi
+	mov	$a_runInstrumentedCode, %eax
+	cmovnz	%edi, %eax
+	ret
+	/* There is a serial-mode transaction, so abort (see htm_abort()
+	   regarding the abort code).  */
+1:	xabort	$0xff
+.Ltxn_abort:
+	/* If it might make sense to retry the HTM fast path, let the C++
+	   code decide.  */
+	testl	$(_XABORT_RETRY|_XABORT_EXPLICIT), %eax
+	jz	.Lno_htm
+	/* Store edi for future HTM fast path retries.  We use a stack slot
+	   lower than the jmpbuf so that the jmpbuf's rip field will overlap
+	   with the proper return address on the stack.  */
+	movl	%edi, -64(%rsp)
+	orl	$pr_HTMRetryableAbort, %edi
+	/* Let the C++ code handle the retry policy.  */
+.Lno_htm:
+#endif
 	leaq	8(%rsp), %rax
-	subq	$56, %rsp
-	cfi_def_cfa_offset(64)
-	movq	%rax, (%rsp)
-	movq	%rbx, 8(%rsp)
-	movq	%rbp, 16(%rsp)
-	movq	%r12, 24(%rsp)
-	movq	%r13, 32(%rsp)
-	movq	%r14, 40(%rsp)
-	movq	%r15, 48(%rsp)
-	movq	%rsp, %rsi
+	subq	$72, %rsp
+	cfi_adjust_cfa_offset(72)
+	/* Save the jmpbuf for any non-HTM-fastpath execution method.
+	   Because rsp-based addressing is 1 byte larger and we've got rax
+	   handy, use it.  */
+	movq	%rax, -64(%rax)
+	movq	%rbx, -56(%rax)
+	movq	%rbp, -48(%rax)
+	movq	%r12, -40(%rax)
+	movq	%r13, -32(%rax)
+	movq	%r14, -24(%rax)
+	movq	%r15, -16(%rax)
+	leaq	-64(%rax), %rsi
 	call	SYM(GTM_begin_transaction)
-	addq	$56, %rsp
-	cfi_def_cfa_offset(8)
+	addq	$72, %rsp
+	cfi_adjust_cfa_offset(-72)
+#ifdef HAVE_AS_RTM
+	/* If a_tryHTMFastPath was returned, then we need to retry the
+	   fast path.  We also restore edi and set pr_HTMRetriedAfterAbort
+	   to state that we have retried the fast path already (it's harmless
+	   if this bit is set even if we don't retry the fast path because it
+	   is checked iff pr_HTMRetryableAbort is set).  */
+	cmpl	$a_tryHTMFastPath, %eax
+	jnz	2f
+	movl	-64(%rsp), %edi
+	orl	$pr_HTMRetriedAfterAbort, %edi
+	jmp	.Lhtm_fastpath
+2:
+#endif
 #else
 	leal	4(%esp), %ecx
 	movl	4(%esp), %eax
diff --git a/libitm/config/x86/target.h b/libitm/config/x86/target.h
index 063c09e..65efb31 100644
--- a/libitm/config/x86/target.h
+++ b/libitm/config/x86/target.h
@@ -70,6 +70,10 @@ cpu_relax (void)
 // See gtm_thread::begin_transaction for how these functions are used.
 #ifdef HAVE_AS_RTM
 #define USE_HTM_FASTPATH
+#ifdef __x86_64__
+// Use the custom fastpath in ITM_beginTransaction.
+#define HTM_CUSTOM_FASTPATH
+#endif
 
 static inline bool
 htm_available ()
diff --git a/libitm/libitm.h b/libitm/libitm.h
index 20ac59a..4bee0a2 100644
--- a/libitm/libitm.h
+++ b/libitm/libitm.h
@@ -72,7 +72,9 @@ typedef enum
     inIrrevocableTransaction
 } _ITM_howExecuting;
 
-/* Values to describe properties of code, passed in to beginTransaction */
+/* Values to describe properties of code, passed in to beginTransaction.
+   Some of these constants are duplicated in some of the ITM_beginTransaction
+   implementations, so update those too when applying any changes.  */
 typedef enum
 {
    pr_instrumentedCode		= 0x0001,
@@ -95,10 +97,16 @@ typedef enum
    pr_exceptionBlock		= 0x1000,
    pr_hasElse			= 0x2000,
    pr_readOnly			= 0x4000,
-   pr_hasNoSimpleReads		= 0x400000
+   pr_hasNoSimpleReads		= 0x400000,
+   /* These are not part of the ABI but used for custom HTM fast paths.  See
+      ITM_beginTransaction and gtm_thread::begin_transaction.  */
+   pr_HTMRetryableAbort		= 0x800000,
+   pr_HTMRetriedAfterAbort	= 0x1000000
 } _ITM_codeProperties;
 
-/* Result from startTransaction that describes what actions to take.  */
+/* Result from startTransaction that describes what actions to take.
+   Some of these constants are duplicated in some of the ITM_beginTransaction
+   implementations, so update those too when applying any changes.  */
 typedef enum
 {
    a_runInstrumentedCode       = 0x01,
@@ -106,6 +114,7 @@ typedef enum
    a_saveLiveVariables         = 0x04,
    a_restoreLiveVariables      = 0x08,
    a_abortTransaction          = 0x10,
+   a_tryHTMFastPath            = 0x20
 } _ITM_actions;
 
 typedef struct
diff --git a/libitm/libitm_i.h b/libitm/libitm_i.h
index 421b349..e965caf 100644
--- a/libitm/libitm_i.h
+++ b/libitm/libitm_i.h
@@ -87,6 +87,14 @@ enum gtm_restart_reason
 #include "dispatch.h"
 #include "containers.h"
 
+#ifdef __USER_LABEL_PREFIX__
+# define UPFX UPFX1(__USER_LABEL_PREFIX__)
+# define UPFX1(t) UPFX2(t)
+# define UPFX2(t) #t
+#else
+# define UPFX
+#endif
+
 namespace GTM HIDDEN {
 
 // This type is private to alloc.c, but needs to be defined so that
@@ -230,6 +238,7 @@ struct gtm_thread
   // be used for the next iteration of the transaction.
   // Only restart_total is reset to zero when the transaction commits, the
   // other counters are total values for all previously executed transactions.
+  // restart_total is also used by the HTM fastpath in a different way.
   uint32_t restart_reason[NUM_RESTARTS];
   uint32_t restart_total;
 
@@ -247,7 +256,9 @@ struct gtm_thread
   // The lock that provides access to serial mode.  Non-serialized
   // transactions acquire read locks; a serialized transaction aquires
   // a write lock.
-  static gtm_rwlock serial_lock;
+  // Accessed from assembly language, thus the "asm" specifier on
+  // the name, avoiding complex name mangling.
+  static gtm_rwlock serial_lock __asm__(UPFX "gtm_serial_lock");
 
   // The head of the list of all threads' transactions.
   static gtm_thread *list_of_threads;
@@ -277,15 +288,8 @@ struct gtm_thread
 
   // Invoked from assembly language, thus the "asm" specifier on
   // the name, avoiding complex name mangling.
-#ifdef __USER_LABEL_PREFIX__
-#define UPFX1(t) UPFX(t)
-#define UPFX(t) #t
-  static uint32_t begin_transaction(uint32_t, const gtm_jmpbuf *)
-	__asm__(UPFX1(__USER_LABEL_PREFIX__) "GTM_begin_transaction") ITM_REGPARM;
-#else
   static uint32_t begin_transaction(uint32_t, const gtm_jmpbuf *)
-	__asm__("GTM_begin_transaction") ITM_REGPARM;
-#endif
+	__asm__(UPFX "GTM_begin_transaction") ITM_REGPARM;
   // In eh_cpp.cc
   void revert_cpp_exceptions (gtm_transaction_cp *cp = 0);
 
@@ -338,7 +342,9 @@ extern gtm_cacheline_mask gtm_mask_stack(gtm_cacheline *, gtm_cacheline_mask);
 
 // Control variable for the HTM fastpath that uses serial mode as fallback.
 // Non-zero if the HTM fastpath is enabled. See gtm_thread::begin_transaction.
-extern uint32_t htm_fastpath;
+// Accessed from assembly language, thus the "asm" specifier on
+// the name, avoiding complex name mangling.
+extern uint32_t htm_fastpath __asm__(UPFX "gtm_htm_fastpath");
 
 } // namespace GTM
 
