arm.c (arm_override_options): Add unaligned_access support.

	gcc/
	* config/arm/arm.c (arm_override_options): Add unaligned_access
	support.
	(arm_file_start): Emit attribute for unaligned access as
	appropriate.
	* config/arm/arm.md (UNSPEC_UNALIGNED_LOAD)
	(UNSPEC_UNALIGNED_STORE): Add constants for unspecs.
	(insv, extzv): Add unaligned-access support.
	(extv): Change to expander. Likewise.
	(extzv_t1, extv_regsi): Add helpers.
	(unaligned_loadsi, unaligned_loadhis, unaligned_loadhiu)
	(unaligned_storesi, unaligned_storehi): New.
	(*extv_reg): New (previous extv implementation).
	* config/arm/arm.opt (munaligned_access): Add option.
	* config/arm/constraints.md (Uw): New constraint.
	* expmed.c (store_bit_field_1): Adjust bitfield numbering according
	to size of access, not size of unit, when BITS_BIG_ENDIAN !=
	BYTES_BIG_ENDIAN. Don't use bitfield accesses for
	volatile accesses when -fstrict-volatile-bitfields is in effect.
	(extract_bit_field_1): Likewise.

From-SVN: r178852

6 files changed, 352 insertions, 48 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 941abc4..1770b73 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,25 @@
+2011-09-14  Julian Brown  <julian@codesourcery.com>
+
+	* config/arm/arm.c (arm_override_options): Add unaligned_access
+	support.
+	(arm_file_start): Emit attribute for unaligned access as
+	appropriate.
+	* config/arm/arm.md (UNSPEC_UNALIGNED_LOAD)
+	(UNSPEC_UNALIGNED_STORE): Add constants for unspecs.
+	(insv, extzv): Add unaligned-access support.
+	(extv): Change to expander. Likewise.
+	(extzv_t1, extv_regsi): Add helpers.
+	(unaligned_loadsi, unaligned_loadhis, unaligned_loadhiu)
+	(unaligned_storesi, unaligned_storehi): New.
+	(*extv_reg): New (previous extv implementation).
+	* config/arm/arm.opt (munaligned_access): Add option.
+	* config/arm/constraints.md (Uw): New constraint.
+	* expmed.c (store_bit_field_1): Adjust bitfield numbering according
+	to size of access, not size of unit, when BITS_BIG_ENDIAN !=
+	BYTES_BIG_ENDIAN. Don't use bitfield accesses for
+	volatile accesses when -fstrict-volatile-bitfields is in effect.
+	(extract_bit_field_1): Likewise.
+
 2011-09-14  Richard Sandiford  <richard.sandiford@linaro.org>
 
 	* simplify-rtx.c (simplify_subreg): Check that the inner mode is
diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 7549562..b45f939 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -1915,6 +1915,28 @@ arm_option_override (void)
 	fix_cm3_ldrd = 0;
     }
 
+  /* Enable -munaligned-access by default for
+     - all ARMv6 architecture-based processors
+     - ARMv7-A, ARMv7-R, and ARMv7-M architecture-based processors.
+
+     Disable -munaligned-access by default for
+     - all pre-ARMv6 architecture-based processors
+     - ARMv6-M architecture-based processors.  */
+
+  if (unaligned_access == 2)
+    {
+      if (arm_arch6 && (arm_arch_notm || arm_arch7))
+	unaligned_access = 1;
+      else
+	unaligned_access = 0;
+    }
+  else if (unaligned_access == 1
+	   && !(arm_arch6 && (arm_arch_notm || arm_arch7)))
+    {
+      warning (0, "target CPU does not support unaligned accesses");
+      unaligned_access = 0;
+    }
+
   if (TARGET_THUMB1 && flag_schedule_insns)
     {
       /* Don't warn since it's on by default in -O2.  */
@@ -22274,6 +22296,10 @@ arm_file_start (void)
 	val = 6;
       asm_fprintf (asm_out_file, "\t.eabi_attribute 30, %d\n", val);
 
+      /* Tag_CPU_unaligned_access.  */
+      asm_fprintf (asm_out_file, "\t.eabi_attribute 34, %d\n",
+		   unaligned_access);
+
       /* Tag_ABI_FP_16bit_format.  */
       if (arm_fp16_format)
 	asm_fprintf (asm_out_file, "\t.eabi_attribute 38, %d\n",
diff --git a/gcc/config/arm/arm.md b/gcc/config/arm/arm.md
index 0732ba1..bb9e328 100644
--- a/gcc/config/arm/arm.md
+++ b/gcc/config/arm/arm.md
@@ -112,6 +112,10 @@
   UNSPEC_SYMBOL_OFFSET  ; The offset of the start of the symbol from
                         ; another symbolic address.
   UNSPEC_MEMORY_BARRIER ; Represent a memory barrier.
+  UNSPEC_UNALIGNED_LOAD	; Used to represent ldr/ldrh instructions that access
+			; unaligned locations, on architectures which support
+			; that.
+  UNSPEC_UNALIGNED_STORE ; Same for str/strh.
 ])
 
 ;; UNSPEC_VOLATILE Usage:
@@ -2481,10 +2485,10 @@
 ;;; this insv pattern, so this pattern needs to be reevalutated.
 
 (define_expand "insv"
-  [(set (zero_extract:SI (match_operand:SI 0 "s_register_operand" "")
-                         (match_operand:SI 1 "general_operand" "")
-                         (match_operand:SI 2 "general_operand" ""))
-        (match_operand:SI 3 "reg_or_int_operand" ""))]
+  [(set (zero_extract (match_operand 0 "nonimmediate_operand" "")
+                      (match_operand 1 "general_operand" "")
+                      (match_operand 2 "general_operand" ""))
+        (match_operand 3 "reg_or_int_operand" ""))]
   "TARGET_ARM || arm_arch_thumb2"
   "
   {
@@ -2495,35 +2499,70 @@
 
     if (arm_arch_thumb2)
       {
-	bool use_bfi = TRUE;
-
-	if (GET_CODE (operands[3]) == CONST_INT)
+        if (unaligned_access && MEM_P (operands[0])
+	    && s_register_operand (operands[3], GET_MODE (operands[3]))
+	    && (width == 16 || width == 32) && (start_bit % BITS_PER_UNIT) == 0)
 	  {
-	    HOST_WIDE_INT val = INTVAL (operands[3]) & mask;
+	    rtx base_addr;
+
+	    if (BYTES_BIG_ENDIAN)
+	      start_bit = GET_MODE_BITSIZE (GET_MODE (operands[3])) - width
+			  - start_bit;
 
-	    if (val == 0)
+	    if (width == 32)
 	      {
-		emit_insn (gen_insv_zero (operands[0], operands[1],
-					  operands[2]));
-		DONE;
+	        base_addr = adjust_address (operands[0], SImode,
+					    start_bit / BITS_PER_UNIT);
+		emit_insn (gen_unaligned_storesi (base_addr, operands[3]));
 	      }
+	    else
+	      {
+	        rtx tmp = gen_reg_rtx (HImode);
 
-	    /* See if the set can be done with a single orr instruction.  */
-	    if (val == mask && const_ok_for_arm (val << start_bit))
-	      use_bfi = FALSE;
+	        base_addr = adjust_address (operands[0], HImode,
+					    start_bit / BITS_PER_UNIT);
+		emit_move_insn (tmp, gen_lowpart (HImode, operands[3]));
+		emit_insn (gen_unaligned_storehi (base_addr, tmp));
+	      }
+	    DONE;
 	  }
-	  
-	if (use_bfi)
+	else if (s_register_operand (operands[0], GET_MODE (operands[0])))
 	  {
-	    if (GET_CODE (operands[3]) != REG)
-	      operands[3] = force_reg (SImode, operands[3]);
+	    bool use_bfi = TRUE;
 
-	    emit_insn (gen_insv_t2 (operands[0], operands[1], operands[2],
-				    operands[3]));
-	    DONE;
+	    if (GET_CODE (operands[3]) == CONST_INT)
+	      {
+		HOST_WIDE_INT val = INTVAL (operands[3]) & mask;
+
+		if (val == 0)
+		  {
+		    emit_insn (gen_insv_zero (operands[0], operands[1],
+					      operands[2]));
+		    DONE;
+		  }
+
+		/* See if the set can be done with a single orr instruction.  */
+		if (val == mask && const_ok_for_arm (val << start_bit))
+		  use_bfi = FALSE;
+	      }
+
+	    if (use_bfi)
+	      {
+		if (GET_CODE (operands[3]) != REG)
+		  operands[3] = force_reg (SImode, operands[3]);
+
+		emit_insn (gen_insv_t2 (operands[0], operands[1], operands[2],
+					operands[3]));
+		DONE;
+	      }
 	  }
+	else
+	  FAIL;
       }
 
+    if (!s_register_operand (operands[0], GET_MODE (operands[0])))
+      FAIL;
+
     target = copy_rtx (operands[0]);
     /* Avoid using a subreg as a subtarget, and avoid writing a paradoxical 
        subreg as the final target.  */
@@ -3715,12 +3754,10 @@
 ;; to reduce register pressure later on.
 
 (define_expand "extzv"
-  [(set (match_dup 4)
-	(ashift:SI (match_operand:SI   1 "register_operand" "")
-		   (match_operand:SI   2 "const_int_operand" "")))
-   (set (match_operand:SI              0 "register_operand" "")
-	(lshiftrt:SI (match_dup 4)
-		     (match_operand:SI 3 "const_int_operand" "")))]
+  [(set (match_operand 0 "s_register_operand" "")
+	(zero_extract (match_operand 1 "nonimmediate_operand" "")
+		      (match_operand 2 "const_int_operand" "")
+		      (match_operand 3 "const_int_operand" "")))]
   "TARGET_THUMB1 || arm_arch_thumb2"
   "
   {
@@ -3729,10 +3766,57 @@
     
     if (arm_arch_thumb2)
       {
-	emit_insn (gen_extzv_t2 (operands[0], operands[1], operands[2],
-				 operands[3]));
-	DONE;
+	HOST_WIDE_INT width = INTVAL (operands[2]);
+	HOST_WIDE_INT bitpos = INTVAL (operands[3]);
+
+	if (unaligned_access && MEM_P (operands[1])
+	    && (width == 16 || width == 32) && (bitpos % BITS_PER_UNIT) == 0)
+	  {
+	    rtx base_addr;
+
+	    if (BYTES_BIG_ENDIAN)
+	      bitpos = GET_MODE_BITSIZE (GET_MODE (operands[0])) - width
+		       - bitpos;
+
+	    if (width == 32)
+              {
+		base_addr = adjust_address (operands[1], SImode,
+					    bitpos / BITS_PER_UNIT);
+		emit_insn (gen_unaligned_loadsi (operands[0], base_addr));
+              }
+	    else
+              {
+		rtx dest = operands[0];
+		rtx tmp = gen_reg_rtx (SImode);
+
+		/* We may get a paradoxical subreg here.  Strip it off.  */
+		if (GET_CODE (dest) == SUBREG
+		    && GET_MODE (dest) == SImode
+		    && GET_MODE (SUBREG_REG (dest)) == HImode)
+		  dest = SUBREG_REG (dest);
+
+		if (GET_MODE_BITSIZE (GET_MODE (dest)) != width)
+		  FAIL;
+
+		base_addr = adjust_address (operands[1], HImode,
+					    bitpos / BITS_PER_UNIT);
+		emit_insn (gen_unaligned_loadhiu (tmp, base_addr));
+		emit_move_insn (gen_lowpart (SImode, dest), tmp);
+	      }
+	    DONE;
+	  }
+	else if (s_register_operand (operands[1], GET_MODE (operands[1])))
+	  {
+	    emit_insn (gen_extzv_t2 (operands[0], operands[1], operands[2],
+				     operands[3]));
+	    DONE;
+	  }
+	else
+	  FAIL;
       }
+    
+    if (!s_register_operand (operands[1], GET_MODE (operands[1])))
+      FAIL;
 
     operands[3] = GEN_INT (rshift);
     
@@ -3742,12 +3826,154 @@
         DONE;
       }
       
-    operands[2] = GEN_INT (lshift);
-    operands[4] = gen_reg_rtx (SImode);
+    emit_insn (gen_extzv_t1 (operands[0], operands[1], GEN_INT (lshift),
+			     operands[3], gen_reg_rtx (SImode)));
+    DONE;
   }"
 )
 
-(define_insn "extv"
+;; Helper for extzv, for the Thumb-1 register-shifts case.
+
+(define_expand "extzv_t1"
+  [(set (match_operand:SI 4 "s_register_operand" "")
+	(ashift:SI (match_operand:SI 1 "nonimmediate_operand" "")
+		   (match_operand:SI 2 "const_int_operand" "")))
+   (set (match_operand:SI 0 "s_register_operand" "")
+	(lshiftrt:SI (match_dup 4)
+		     (match_operand:SI 3 "const_int_operand" "")))]
+  "TARGET_THUMB1"
+  "")
+
+(define_expand "extv"
+  [(set (match_operand 0 "s_register_operand" "")
+	(sign_extract (match_operand 1 "nonimmediate_operand" "")
+		      (match_operand 2 "const_int_operand" "")
+		      (match_operand 3 "const_int_operand" "")))]
+  "arm_arch_thumb2"
+{
+  HOST_WIDE_INT width = INTVAL (operands[2]);
+  HOST_WIDE_INT bitpos = INTVAL (operands[3]);
+
+  if (unaligned_access && MEM_P (operands[1]) && (width == 16 || width == 32)
+      && (bitpos % BITS_PER_UNIT)  == 0)
+    {
+      rtx base_addr;
+      
+      if (BYTES_BIG_ENDIAN)
+	bitpos = GET_MODE_BITSIZE (GET_MODE (operands[0])) - width - bitpos;
+      
+      if (width == 32)
+        {
+	  base_addr = adjust_address (operands[1], SImode,
+				      bitpos / BITS_PER_UNIT);
+	  emit_insn (gen_unaligned_loadsi (operands[0], base_addr));
+        }
+      else
+        {
+	  rtx dest = operands[0];
+	  rtx tmp = gen_reg_rtx (SImode);
+	  
+	  /* We may get a paradoxical subreg here.  Strip it off.  */
+	  if (GET_CODE (dest) == SUBREG
+	      && GET_MODE (dest) == SImode
+	      && GET_MODE (SUBREG_REG (dest)) == HImode)
+	    dest = SUBREG_REG (dest);
+	  
+	  if (GET_MODE_BITSIZE (GET_MODE (dest)) != width)
+	    FAIL;
+	  
+	  base_addr = adjust_address (operands[1], HImode,
+				      bitpos / BITS_PER_UNIT);
+	  emit_insn (gen_unaligned_loadhis (tmp, base_addr));
+	  emit_move_insn (gen_lowpart (SImode, dest), tmp);
+	}
+
+      DONE;
+    }
+  else if (!s_register_operand (operands[1], GET_MODE (operands[1])))
+    FAIL;
+  else if (GET_MODE (operands[0]) == SImode
+	   && GET_MODE (operands[1]) == SImode)
+    {
+      emit_insn (gen_extv_regsi (operands[0], operands[1], operands[2],
+				 operands[3]));
+      DONE;
+    }
+
+  FAIL;
+})
+
+; Helper to expand register forms of extv with the proper modes.
+
+(define_expand "extv_regsi"
+  [(set (match_operand:SI 0 "s_register_operand" "")
+	(sign_extract:SI (match_operand:SI 1 "s_register_operand" "")
+			 (match_operand 2 "const_int_operand" "")
+			 (match_operand 3 "const_int_operand" "")))]
+  ""
+{
+})
+
+; ARMv6+ unaligned load/store instructions (used for packed structure accesses).
+
+(define_insn "unaligned_loadsi"
+  [(set (match_operand:SI 0 "s_register_operand" "=l,r")
+	(unspec:SI [(match_operand:SI 1 "memory_operand" "Uw,m")]
+		   UNSPEC_UNALIGNED_LOAD))]
+  "unaligned_access && TARGET_32BIT"
+  "ldr%?\t%0, %1\t@ unaligned"
+  [(set_attr "arch" "t2,any")
+   (set_attr "length" "2,4")
+   (set_attr "predicable" "yes")
+   (set_attr "type" "load1")])
+
+(define_insn "unaligned_loadhis"
+  [(set (match_operand:SI 0 "s_register_operand" "=l,r")
+	(sign_extend:SI
+	  (unspec:HI [(match_operand:HI 1 "memory_operand" "Uw,m")]
+		     UNSPEC_UNALIGNED_LOAD)))]
+  "unaligned_access && TARGET_32BIT"
+  "ldr%(sh%)\t%0, %1\t@ unaligned"
+  [(set_attr "arch" "t2,any")
+   (set_attr "length" "2,4")
+   (set_attr "predicable" "yes")
+   (set_attr "type" "load_byte")])
+
+(define_insn "unaligned_loadhiu"
+  [(set (match_operand:SI 0 "s_register_operand" "=l,r")
+	(zero_extend:SI
+	  (unspec:HI [(match_operand:HI 1 "memory_operand" "Uw,m")]
+		     UNSPEC_UNALIGNED_LOAD)))]
+  "unaligned_access && TARGET_32BIT"
+  "ldr%(h%)\t%0, %1\t@ unaligned"
+  [(set_attr "arch" "t2,any")
+   (set_attr "length" "2,4")
+   (set_attr "predicable" "yes")
+   (set_attr "type" "load_byte")])
+
+(define_insn "unaligned_storesi"
+  [(set (match_operand:SI 0 "memory_operand" "=Uw,m")
+	(unspec:SI [(match_operand:SI 1 "s_register_operand" "l,r")]
+		   UNSPEC_UNALIGNED_STORE))]
+  "unaligned_access && TARGET_32BIT"
+  "str%?\t%1, %0\t@ unaligned"
+  [(set_attr "arch" "t2,any")
+   (set_attr "length" "2,4")
+   (set_attr "predicable" "yes")
+   (set_attr "type" "store1")])
+
+(define_insn "unaligned_storehi"
+  [(set (match_operand:HI 0 "memory_operand" "=Uw,m")
+	(unspec:HI [(match_operand:HI 1 "s_register_operand" "l,r")]
+		   UNSPEC_UNALIGNED_STORE))]
+  "unaligned_access && TARGET_32BIT"
+  "str%(h%)\t%1, %0\t@ unaligned"
+  [(set_attr "arch" "t2,any")
+   (set_attr "length" "2,4")
+   (set_attr "predicable" "yes")
+   (set_attr "type" "store1")])
+
+(define_insn "*extv_reg"
   [(set (match_operand:SI 0 "s_register_operand" "=r")
 	(sign_extract:SI (match_operand:SI 1 "s_register_operand" "r")
                          (match_operand:SI 2 "const_int_operand" "M")
diff --git a/gcc/config/arm/arm.opt b/gcc/config/arm/arm.opt
index 4d29e2f..e33b460 100644
--- a/gcc/config/arm/arm.opt
+++ b/gcc/config/arm/arm.opt
@@ -253,3 +253,7 @@ mfix-cortex-m3-ldrd
 Target Report Var(fix_cm3_ldrd) Init(2)
 Avoid overlapping destination and address registers on LDRD instructions
 that may trigger Cortex-M3 errata.
+
+munaligned-access
+Target Report Var(unaligned_access) Init(2)
+Enable unaligned word and halfword accesses to packed data.
diff --git a/gcc/config/arm/constraints.md b/gcc/config/arm/constraints.md
index 22df032..d8ce982 100644
--- a/gcc/config/arm/constraints.md
+++ b/gcc/config/arm/constraints.md
@@ -36,7 +36,7 @@
 ;; The following memory constraints have been used:
 ;; in ARM/Thumb-2 state: Q, Ut, Uv, Uy, Un, Um, Us
 ;; in ARM state: Uq
-;; in Thumb state: Uu
+;; in Thumb state: Uu, Uw
 
 
 (define_register_constraint "f" "TARGET_ARM ? FPA_REGS : NO_REGS"
@@ -353,6 +353,19 @@
 		   && thumb1_legitimate_address_p (GET_MODE (op), XEXP (op, 0),
 						   0)")))
 
+; The 16-bit post-increment LDR/STR accepted by thumb1_legitimate_address_p
+; are actually LDM/STM instructions, so cannot be used to access unaligned
+; data.
+(define_memory_constraint "Uw"
+ "@internal
+  In Thumb state an address that is valid in 16bit encoding, and that can be
+  used for unaligned accesses."
+ (and (match_code "mem")
+      (match_test "TARGET_THUMB
+		   && thumb1_legitimate_address_p (GET_MODE (op), XEXP (op, 0),
+						   0)
+		   && GET_CODE (XEXP (op, 0)) != POST_INC")))
+
 ;; We used to have constraint letters for S and R in ARM state, but
 ;; all uses of these now appear to have been removed.
 
diff --git a/gcc/expmed.c b/gcc/expmed.c
index 0047cd0..1528fbb 100644
--- a/gcc/expmed.c
+++ b/gcc/expmed.c
@@ -620,6 +620,10 @@ store_bit_field_1 (rtx str_rtx, unsigned HOST_WIDE_INT bitsize,
       && GET_MODE (value) != BLKmode
       && bitsize > 0
       && GET_MODE_BITSIZE (op_mode) >= bitsize
+      /* Do not use insv for volatile bitfields when
+         -fstrict-volatile-bitfields is in effect.  */
+      && !(MEM_P (op0) && MEM_VOLATILE_P (op0)
+	   && flag_strict_volatile_bitfields > 0)
       && ! ((REG_P (op0) || GET_CODE (op0) == SUBREG)
 	    && (bitsize + bitpos > GET_MODE_BITSIZE (op_mode))))
     {
@@ -659,19 +663,21 @@ store_bit_field_1 (rtx str_rtx, unsigned HOST_WIDE_INT bitsize,
 	  copy_back = true;
 	}
 
-      /* On big-endian machines, we count bits from the most significant.
-	 If the bit field insn does not, we must invert.  */
-
-      if (BITS_BIG_ENDIAN != BYTES_BIG_ENDIAN)
-	xbitpos = unit - bitsize - xbitpos;
-
       /* We have been counting XBITPOS within UNIT.
 	 Count instead within the size of the register.  */
-      if (BITS_BIG_ENDIAN && !MEM_P (xop0))
+      if (BYTES_BIG_ENDIAN && !MEM_P (xop0))
 	xbitpos += GET_MODE_BITSIZE (op_mode) - unit;
 
       unit = GET_MODE_BITSIZE (op_mode);
 
+      /* If BITS_BIG_ENDIAN is zero on a BYTES_BIG_ENDIAN machine, we count
+         "backwards" from the size of the unit we are inserting into.
+	 Otherwise, we count bits from the most significant on a
+	 BYTES/BITS_BIG_ENDIAN machine.  */
+
+      if (BITS_BIG_ENDIAN != BYTES_BIG_ENDIAN)
+	xbitpos = unit - bitsize - xbitpos;
+
       /* Convert VALUE to op_mode (which insv insn wants) in VALUE1.  */
       value1 = value;
       if (GET_MODE (value) != op_mode)
@@ -1507,6 +1513,10 @@ extract_bit_field_1 (rtx str_rtx, unsigned HOST_WIDE_INT bitsize,
   if (ext_mode != MAX_MACHINE_MODE
       && bitsize > 0
       && GET_MODE_BITSIZE (ext_mode) >= bitsize
+      /* Do not use extv/extzv for volatile bitfields when
+         -fstrict-volatile-bitfields is in effect.  */
+      && !(MEM_P (op0) && MEM_VOLATILE_P (op0)
+	   && flag_strict_volatile_bitfields > 0)
       /* If op0 is a register, we need it in EXT_MODE to make it
 	 acceptable to the format of ext(z)v.  */
       && !(GET_CODE (op0) == SUBREG && GET_MODE (op0) != ext_mode)
@@ -1528,17 +1538,20 @@ extract_bit_field_1 (rtx str_rtx, unsigned HOST_WIDE_INT bitsize,
 	/* Get ref to first byte containing part of the field.  */
 	xop0 = adjust_address (xop0, byte_mode, xoffset);
 
-      /* On big-endian machines, we count bits from the most significant.
-	 If the bit field insn does not, we must invert.  */
-      if (BITS_BIG_ENDIAN != BYTES_BIG_ENDIAN)
-	xbitpos = unit - bitsize - xbitpos;
-
       /* Now convert from counting within UNIT to counting in EXT_MODE.  */
-      if (BITS_BIG_ENDIAN && !MEM_P (xop0))
+      if (BYTES_BIG_ENDIAN && !MEM_P (xop0))
 	xbitpos += GET_MODE_BITSIZE (ext_mode) - unit;
 
       unit = GET_MODE_BITSIZE (ext_mode);
 
+      /* If BITS_BIG_ENDIAN is zero on a BYTES_BIG_ENDIAN machine, we count
+         "backwards" from the size of the unit we are extracting from.
+	 Otherwise, we count bits from the most significant on a
+	 BYTES/BITS_BIG_ENDIAN machine.  */
+
+      if (BITS_BIG_ENDIAN != BYTES_BIG_ENDIAN)
+	xbitpos = unit - bitsize - xbitpos;
+
       if (xtarget == 0)
 	xtarget = xspec_target = gen_reg_rtx (tmode);