RISC-V: Add sifive-7 pipeline description.

	* config/riscv/generic.md (generic_alu, generic_load, generic_store)
	(generic_xfer, generic_branch, generic_imul, generic_idivsi)
	(generic_idivdi, generic_fmul_single, generic_fmul_double)
	(generic_fdiv, generic_fsqrt): Add check for generic tune.
	(generic_alu): Add auipc to type list.
	* config/riscv/riscv-opts.h (enum riscv_microarchitecture_type): New.
	(riscv_microarchitecture): Declare.
	* config/riscv/riscv-protos.h (riscv_store_data_bypass_p): Declare.
	* config/riscv/riscv.c (struct riscv_cpu_info): Add microarchitecture
	field.
	(riscv_microarchitecture): New.
	(sifive_7_tune_info): New.
	(riscv_cpu_info_table): Add microarchitecture value for rocket and
	size.  Add sifive-3-series, sifive-5-series, and sifive-7-series
	entries.
	(riscv_store_data_bypass_p): New.
	(riscv_option_override): Set riscv_microarchitecture from
	cpu->microarchitecture.
	* config/riscv/riscv.md: Include sifive-7.md.
	(type): Add auipc.
	(tune): New.
	(auipc<mode>): Change type to auipc.
	(restore_stack_nonlocal): New.
	* config/riscv/sifive-7.md: New.
	* doc/invoke.texi (RISC-V Options): Update mtune docs.

Co-Authored-By: Jim Wilson <jimw@sifive.com>
From-SVN: r269954

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index c83f88a949387affdd4b86e88348e319b83a35c2..a298f878bbb36929d6ac221baadf24dec4c81363 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,32 @@
+2019-03-26  Andrew Waterman  <andrew@sifive.com>
+           Jim Wilson  <jimw@sifive.com>
+
+       * config/riscv/generic.md (generic_alu, generic_load, generic_store)
+       (generic_xfer, generic_branch, generic_imul, generic_idivsi)
+       (generic_idivdi, generic_fmul_single, generic_fmul_double)
+       (generic_fdiv, generic_fsqrt): Add check for generic tune.
+       (generic_alu): Add auipc to type list.
+       * config/riscv/riscv-opts.h (enum riscv_microarchitecture_type): New.
+       (riscv_microarchitecture): Declare.
+       * config/riscv/riscv-protos.h (riscv_store_data_bypass_p): Declare.
+       * config/riscv/riscv.c (struct riscv_cpu_info): Add microarchitecture
+       field.
+       (riscv_microarchitecture): New.
+       (sifive_7_tune_info): New.
+       (riscv_cpu_info_table): Add microarchitecture value for rocket and
+       size.  Add sifive-3-series, sifive-5-series, and sifive-7-series
+       entries.
+       (riscv_store_data_bypass_p): New.
+       (riscv_option_override): Set riscv_microarchitecture from
+       cpu->microarchitecture.
+       * config/riscv/riscv.md: Include sifive-7.md.
+       (type): Add auipc.
+       (tune): New.
+       (auipc<mode>): Change type to auipc.
+       (restore_stack_nonlocal): New.
+       * config/riscv/sifive-7.md: New.
+       * doc/invoke.texi (RISC-V Options): Update mtune docs.
+
 2019-03-26  Uroš Bizjak  <ubizjak@gmail.com>
 
        PR target/89827

diff --git a/gcc/config/riscv/generic.md b/gcc/config/riscv/generic.md
index 13c198113199729c879a5c877116087a72c785b2..fcd78b990feccbdceeee70df8de2cd1341757744 100644 (file)
--- a/gcc/config/riscv/generic.md
+++ b/gcc/config/riscv/generic.md
@@ -26,53 +26,65 @@
 (define_cpu_unit "fdivsqrt" "pipe0")
 
 (define_insn_reservation "generic_alu" 1
-  (eq_attr "type" "unknown,const,arith,shift,slt,multi,nop,logical,move")
+  (and (eq_attr "tune" "generic")
+       (eq_attr "type" "unknown,const,arith,shift,slt,multi,auipc,nop,logical,move"))
   "alu")
 
 (define_insn_reservation "generic_load" 3
-  (eq_attr "type" "load,fpload")
+  (and (eq_attr "tune" "generic")
+       (eq_attr "type" "load,fpload"))
   "alu")
 
 (define_insn_reservation "generic_store" 1
-  (eq_attr "type" "store,fpstore")
+  (and (eq_attr "tune" "generic")
+       (eq_attr "type" "store,fpstore"))
   "alu")
 
 (define_insn_reservation "generic_xfer" 3
-  (eq_attr "type" "mfc,mtc,fcvt,fmove,fcmp")
+  (and (eq_attr "tune" "generic")
+       (eq_attr "type" "mfc,mtc,fcvt,fmove,fcmp"))
   "alu")
 
 (define_insn_reservation "generic_branch" 1
-  (eq_attr "type" "branch,jump,call")
+  (and (eq_attr "tune" "generic")
+       (eq_attr "type" "branch,jump,call"))
   "alu")
 
 (define_insn_reservation "generic_imul" 10
-  (eq_attr "type" "imul")
+  (and (eq_attr "tune" "generic")
+       (eq_attr "type" "imul"))
   "imuldiv*10")
 
 (define_insn_reservation "generic_idivsi" 34
-  (and (eq_attr "type" "idiv")
-       (eq_attr "mode" "SI"))
+  (and (eq_attr "tune" "generic")
+       (and (eq_attr "type" "idiv")
+           (eq_attr "mode" "SI")))
   "imuldiv*34")
 
 (define_insn_reservation "generic_idivdi" 66
-  (and (eq_attr "type" "idiv")
-       (eq_attr "mode" "DI"))
+  (and (eq_attr "tune" "generic")
+       (and (eq_attr "type" "idiv")
+           (eq_attr "mode" "DI")))
   "imuldiv*66")
 
 (define_insn_reservation "generic_fmul_single" 5
-  (and (eq_attr "type" "fadd,fmul,fmadd")
-       (eq_attr "mode" "SF"))
+  (and (eq_attr "tune" "generic")
+       (and (eq_attr "type" "fadd,fmul,fmadd")
+           (eq_attr "mode" "SF")))
   "alu")
 
 (define_insn_reservation "generic_fmul_double" 7
-  (and (eq_attr "type" "fadd,fmul,fmadd")
-       (eq_attr "mode" "DF"))
+  (and (eq_attr "tune" "generic")
+       (and (eq_attr "type" "fadd,fmul,fmadd")
+           (eq_attr "mode" "DF")))
   "alu")
 
 (define_insn_reservation "generic_fdiv" 20
-  (eq_attr "type" "fdiv")
+  (and (eq_attr "tune" "generic")
+       (eq_attr "type" "fdiv"))
   "fdivsqrt*20")
 
 (define_insn_reservation "generic_fsqrt" 25
-  (eq_attr "type" "fsqrt")
+  (and (eq_attr "tune" "generic")
+       (eq_attr "type" "fsqrt"))
   "fdivsqrt*25")

diff --git a/gcc/config/riscv/riscv-opts.h b/gcc/config/riscv/riscv-opts.h
index a3ab6cec33b497c0b5f8d0976d29595dc3d78fe8..f3031f2e523c8796ed7aac371d55d30bb6ff244f 100644 (file)
--- a/gcc/config/riscv/riscv-opts.h
+++ b/gcc/config/riscv/riscv-opts.h
@@ -39,4 +39,11 @@ enum riscv_code_model {
 };
 extern enum riscv_code_model riscv_cmodel;
 
+/* Keep this list in sync with define_attr "tune" in riscv.md.  */
+enum riscv_microarchitecture_type {
+  generic,
+  sifive_7
+};
+extern enum riscv_microarchitecture_type riscv_microarchitecture;
+
 #endif /* ! GCC_RISCV_OPTS_H */

diff --git a/gcc/config/riscv/riscv-protos.h b/gcc/config/riscv/riscv-protos.h
index b81eff0c04be895194c44ab60c451d6d66552bda..8b510f87df87b0d17d011243f8108e4bb310c5b0 100644 (file)
--- a/gcc/config/riscv/riscv-protos.h
+++ b/gcc/config/riscv/riscv-protos.h
@@ -71,6 +71,7 @@ extern bool riscv_epilogue_uses (unsigned int);
 extern bool riscv_can_use_return_insn (void);
 extern rtx riscv_function_value (const_tree, const_tree, enum machine_mode);
 extern bool riscv_expand_block_move (rtx, rtx, rtx);
+extern bool riscv_store_data_bypass_p (rtx_insn *, rtx_insn *);
 
 /* Routines implemented in riscv-c.c.  */
 void riscv_cpu_cpp_builtins (cpp_reader *);

diff --git a/gcc/config/riscv/riscv.c b/gcc/config/riscv/riscv.c
index d8446f82b9622d054e2fcffe0fb118a1ea703758..9aa0fd2fb71deb1177cbebc2bb9f7f0f843e2521 100644 (file)
--- a/gcc/config/riscv/riscv.c
+++ b/gcc/config/riscv/riscv.c
@@ -226,6 +226,9 @@ struct riscv_cpu_info {
   /* This CPU's canonical name.  */
   const char *name;
 
+  /* Which automaton to use for tuning.  */
+  enum riscv_microarchitecture_type microarchitecture;
+
   /* Tuning parameters for this CPU.  */
   const struct riscv_tune_info *tune_info;
 };
@@ -246,6 +249,9 @@ static int epilogue_cfa_sp_offset;
 /* Which tuning parameters to use.  */
 static const struct riscv_tune_info *tune_info;
 
+/* Which automaton to use for tuning.  */
+enum riscv_microarchitecture_type riscv_microarchitecture;
+
 /* Index R is the smallest register class that contains register R.  */
 const enum reg_class riscv_regno_to_class[FIRST_PSEUDO_REGISTER] = {
   GR_REGS,     GR_REGS,        GR_REGS,        GR_REGS,
@@ -280,6 +286,19 @@ static const struct riscv_tune_info rocket_tune_info = {
   true,                                                /* slow_unaligned_access */
 };
 
+/* Costs to use when optimizing for Sifive 7 Series.  */
+static const struct riscv_tune_info sifive_7_tune_info = {
+  {COSTS_N_INSNS (4), COSTS_N_INSNS (5)},      /* fp_add */
+  {COSTS_N_INSNS (4), COSTS_N_INSNS (5)},      /* fp_mul */
+  {COSTS_N_INSNS (20), COSTS_N_INSNS (20)},    /* fp_div */
+  {COSTS_N_INSNS (4), COSTS_N_INSNS (4)},      /* int_mul */
+  {COSTS_N_INSNS (6), COSTS_N_INSNS (6)},      /* int_div */
+  2,                                           /* issue_rate */
+  4,                                           /* branch_cost */
+  3,                                           /* memory_cost */
+  true,                                                /* slow_unaligned_access */
+};
+
 /* Costs to use when optimizing for size.  */
 static const struct riscv_tune_info optimize_size_tune_info = {
   {COSTS_N_INSNS (1), COSTS_N_INSNS (1)},      /* fp_add */
@@ -316,8 +335,11 @@ static const struct attribute_spec riscv_attribute_table[] =
 
 /* A table describing all the processors GCC knows about.  */
 static const struct riscv_cpu_info riscv_cpu_info_table[] = {
-  { "rocket", &rocket_tune_info },
-  { "size", &optimize_size_tune_info },
+  { "rocket", generic, &rocket_tune_info },
+  { "sifive-3-series", generic, &rocket_tune_info },
+  { "sifive-5-series", generic, &rocket_tune_info },
+  { "sifive-7-series", sifive_7, &sifive_7_tune_info },
+  { "size", generic, &optimize_size_tune_info },
 };
 
 /* Return the riscv_cpu_info entry for the given name string.  */
@@ -4094,6 +4116,93 @@ riscv_can_use_return_insn (void)
          && ! cfun->machine->interrupt_handler_p);
 }
 
+/* Given that there exists at least one variable that is set (produced)
+   by OUT_INSN and read (consumed) by IN_INSN, return true iff
+   IN_INSN represents one or more memory store operations and none of
+   the variables set by OUT_INSN is used by IN_INSN as the address of a
+   store operation.  If either IN_INSN or OUT_INSN does not represent
+   a "single" RTL SET expression (as loosely defined by the
+   implementation of the single_set function) or a PARALLEL with only
+   SETs, CLOBBERs, and USEs inside, this function returns false.
+
+   Borrowed from rs6000, riscv_store_data_bypass_p checks for certain
+   conditions that result in assertion failures in the generic
+   store_data_bypass_p function and returns FALSE in such cases.
+
+   This is required to make -msave-restore work with the sifive-7
+   pipeline description.  */
+
+bool
+riscv_store_data_bypass_p (rtx_insn *out_insn, rtx_insn *in_insn)
+{
+  rtx out_set, in_set;
+  rtx out_pat, in_pat;
+  rtx out_exp, in_exp;
+  int i, j;
+
+  in_set = single_set (in_insn);
+  if (in_set)
+    {
+      if (MEM_P (SET_DEST (in_set)))
+       {
+         out_set = single_set (out_insn);
+         if (!out_set)
+           {
+             out_pat = PATTERN (out_insn);
+             if (GET_CODE (out_pat) == PARALLEL)
+               {
+                 for (i = 0; i < XVECLEN (out_pat, 0); i++)
+                   {
+                     out_exp = XVECEXP (out_pat, 0, i);
+                     if ((GET_CODE (out_exp) == CLOBBER)
+                         || (GET_CODE (out_exp) == USE))
+                       continue;
+                     else if (GET_CODE (out_exp) != SET)
+                       return false;
+                   }
+               }
+           }
+       }
+    }
+  else
+    {
+      in_pat = PATTERN (in_insn);
+      if (GET_CODE (in_pat) != PARALLEL)
+       return false;
+
+      for (i = 0; i < XVECLEN (in_pat, 0); i++)
+       {
+         in_exp = XVECEXP (in_pat, 0, i);
+         if ((GET_CODE (in_exp) == CLOBBER) || (GET_CODE (in_exp) == USE))
+           continue;
+         else if (GET_CODE (in_exp) != SET)
+           return false;
+
+         if (MEM_P (SET_DEST (in_exp)))
+           {
+             out_set = single_set (out_insn);
+             if (!out_set)
+               {
+                 out_pat = PATTERN (out_insn);
+                 if (GET_CODE (out_pat) != PARALLEL)
+                   return false;
+                 for (j = 0; j < XVECLEN (out_pat, 0); j++)
+                   {
+                     out_exp = XVECEXP (out_pat, 0, j);
+                     if ((GET_CODE (out_exp) == CLOBBER)
+                         || (GET_CODE (out_exp) == USE))
+                       continue;
+                     else if (GET_CODE (out_exp) != SET)
+                       return false;
+                   }
+               }
+           }
+       }
+    }
+
+  return store_data_bypass_p (out_insn, in_insn);
+}
+
 /* Implement TARGET_SECONDARY_MEMORY_NEEDED.
 
    When floating-point registers are wider than integer ones, moves between
@@ -4358,6 +4467,7 @@ riscv_option_override (void)
   /* Handle -mtune.  */
   cpu = riscv_parse_cpu (riscv_tune_string ? riscv_tune_string :
                         RISCV_TUNE_STRING_DEFAULT);
+  riscv_microarchitecture = cpu->microarchitecture;
   tune_info = optimize_size ? &optimize_size_tune_info : cpu->tune_info;
 
   /* Use -mtune's setting for slow_unaligned_access, even when optimizing

diff --git a/gcc/config/riscv/riscv.md b/gcc/config/riscv/riscv.md
index 0e968cd2f256944d5813bff45c55d5211544bc7f..e3799a5bdd8b6efed7ab457ac5ef9ed6fa35b393 100644 (file)
--- a/gcc/config/riscv/riscv.md
+++ b/gcc/config/riscv/riscv.md
@@ -156,7 +156,7 @@
 (define_attr "type"
   "unknown,branch,jump,call,load,fpload,store,fpstore,
    mtc,mfc,const,arith,logical,shift,slt,imul,idiv,move,fmove,fadd,fmul,
-   fmadd,fdiv,fcmp,fcvt,fsqrt,multi,nop,ghost"
+   fmadd,fdiv,fcmp,fcvt,fsqrt,multi,auipc,nop,ghost"
   (cond [(eq_attr "got" "load") (const_string "load")
 
         ;; If a doubleword move uses these expensive instructions,
@@ -235,6 +235,12 @@
 ;; Is copying of this instruction disallowed?
 (define_attr "cannot_copy" "no,yes" (const_string "no"))
 
+;; Microarchitectures we know how to tune for.
+;; Keep this in sync with enum riscv_microarchitecture.
+(define_attr "tune"
+  "generic,sifive_7"
+  (const (symbol_ref "((enum attr_tune) riscv_microarchitecture)")))
+
 ;; Describe a user's asm statement.
 (define_asm_attributes
   [(set_attr "type" "multi")])
@@ -1247,7 +1253,7 @@
            UNSPEC_AUIPC))]
   ""
   ".LA%2: auipc\t%0,%h1"
-  [(set_attr "type" "arith")
+  [(set_attr "type" "auipc")
    (set_attr "cannot_copy" "yes")])
 
 ;; Instructions for adding the low 12 bits of an address to a register.
@@ -2422,7 +2428,25 @@
   [(set_attr "length" "0")]
 )
 
+;; This fixes a failure with gcc.c-torture/execute/pr64242.c at -O2 for a
+;; 32-bit target when using -mtune=sifive-7-series.  The first sched pass
+;; runs before register elimination, and we have a non-obvious dependency
+;; between a use of the soft fp and a set of the hard fp.  We fix this by
+;; emitting a clobber using the hard fp between the two insns.
+(define_expand "restore_stack_nonlocal"
+  [(match_operand 0 "register_operand")
+   (match_operand 1 "memory_operand")]
+  ""
+{
+  emit_move_insn (operands[0], operands[1]);
+  /* Prevent the following hard fp restore from being moved before the move
+     insn above which uses a copy of the soft fp reg.  */
+  emit_clobber (gen_rtx_MEM (BLKmode, hard_frame_pointer_rtx));
+  DONE;
+})
+
 (include "sync.md")
 (include "peephole.md")
 (include "pic.md")
 (include "generic.md")
+(include "sifive-7.md")

diff --git a/gcc/config/riscv/sifive-7.md b/gcc/config/riscv/sifive-7.md
new file mode 100644 (file)
index 0000000..d58e01f
--- /dev/null
+++ b/gcc/config/riscv/sifive-7.md
@@ -0,0 +1,120 @@
+(define_automaton "sifive_7")
+
+;; Sifive 7 Series Base Core
+;; This has two pipelines, A (Address) and B (Branch).
+;; Loads, stores, and FP <-> integer moves use the A-pipe.
+;; Branches, MUL/DIV, and FP ops use the B-pipe.
+;; Integer ALU ops can use either pipe.
+
+(define_cpu_unit "sifive_7_A" "sifive_7")
+(define_cpu_unit "sifive_7_B" "sifive_7")
+
+(define_cpu_unit "sifive_7_idiv" "sifive_7")
+(define_cpu_unit "sifive_7_fpu" "sifive_7")
+
+(define_insn_reservation "sifive_7_load" 3
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "load"))
+  "sifive_7_A")
+
+(define_insn_reservation "sifive_7_fpload" 2
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "fpload"))
+  "sifive_7_A")
+
+(define_insn_reservation "sifive_7_store" 1
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "store"))
+  "sifive_7_A")
+
+(define_insn_reservation "sifive_7_fpstore" 1
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "fpstore"))
+  "sifive_7_A")
+
+(define_insn_reservation "sifive_7_branch" 1
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "branch"))
+  "sifive_7_B")
+
+(define_insn_reservation "sifive_7_jump" 1
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "jump,call"))
+  "sifive_7_B")
+
+(define_insn_reservation "sifive_7_mul" 3
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "imul"))
+  "sifive_7_B")
+
+(define_insn_reservation "sifive_7_div" 16
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "idiv"))
+  "sifive_7_B,sifive_7_idiv*15")
+
+(define_insn_reservation "sifive_7_alu" 2
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "unknown,arith,shift,slt,multi,logical,move"))
+  "sifive_7_A|sifive_7_B")
+
+(define_insn_reservation "sifive_7_load_immediate" 1
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "nop,const,auipc"))
+  "sifive_7_A|sifive_7_B")
+
+(define_insn_reservation "sifive_7_sfma" 5
+  (and (eq_attr "tune" "sifive_7")
+       (and (eq_attr "type" "fadd,fmul,fmadd")
+           (eq_attr "mode" "SF")))
+  "sifive_7_B")
+
+(define_insn_reservation "sifive_7_dfma" 7
+  (and (eq_attr "tune" "sifive_7")
+       (and (eq_attr "type" "fadd,fmul,fmadd")
+           (eq_attr "mode" "DF")))
+  "sifive_7_B")
+
+(define_insn_reservation "sifive_7_fp_other" 3
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "fcvt,fcmp,fmove"))
+  "sifive_7_B")
+
+(define_insn_reservation "sifive_7_fdiv_s" 27
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "fdiv,fsqrt")
+       (eq_attr "mode" "SF"))
+  "sifive_7_B,sifive_7_fpu*26")
+
+(define_insn_reservation "sifive_7_fdiv_d" 56
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "fdiv,fsqrt")
+       (eq_attr "mode" "DF"))
+  "sifive_7_B,sifive_7_fpu*55")
+
+(define_insn_reservation "sifive_7_i2f" 3
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "mtc"))
+  "sifive_7_A")
+
+(define_insn_reservation "sifive_7_f2i" 3
+  (and (eq_attr "tune" "sifive_7")
+       (eq_attr "type" "mfc"))
+  "sifive_7_A")
+
+(define_bypass 1 "sifive_7_load,sifive_7_alu,sifive_7_mul,sifive_7_f2i"
+  "sifive_7_alu,sifive_7_branch")
+
+(define_bypass 1 "sifive_7_load,sifive_7_alu,sifive_7_mul,sifive_7_f2i"
+  "sifive_7_store" "riscv_store_data_bypass_p")
+
+(define_bypass 2 "sifive_7_i2f"
+  "sifive_7_sfma,sifive_7_dfma,sifive_7_fp_other,sifive_7_fdiv_s,sifive_7_fdiv_d")
+
+(define_bypass 2 "sifive_7_fp_other"
+  "sifive_7_sfma,sifive_7_dfma,sifive_7_fp_other,sifive_7_fdiv_s,sifive_7_fdiv_d")
+
+(define_bypass 2 "sifive_7_fp_other"
+  "sifive_7_alu,sifive_7_branch")
+
+(define_bypass 2 "sifive_7_fp_other"
+  "sifive_7_store" "riscv_store_data_bypass_p")

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 4735b0ab673794b971e44a3b25e2151c30997285..1787967d753291c4df0839d380bf2f4fa594778e 100644 (file)
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -23779,7 +23779,16 @@ lower-case.  Examples include @samp{rv64i}, @samp{rv32g}, @samp{rv32e}, and
 @item -mtune=@var{processor-string}
 @opindex mtune
 Optimize the output for the given processor, specified by microarchitecture
-name.
+name.  Permissible values for this option are: @samp{rocket},
+@samp{sifive-3-series}, @samp{sifive-5-series}, @samp{sifive-7-series},
+and @samp{size}.
+
+When @option{-mtune=} is not specified, the default is @samp{rocket}.
+
+The @samp{size} choice is not intended for use by end-users.  This is used
+when @option{-Os} is specified.  It overrides the instruction cost info
+provided by @option{-mtune=}, but does not override the pipeline info.  This
+helps reduce code size while still giving good performance.
 
 @item -mpreferred-stack-boundary=@var{num}
 @opindex mpreferred-stack-boundary