Extract the hardcoded values for the minimum PC-relative displacements
into named constants and document them.

gcc/ChangeLog:

        * config/aarch64/aarch64.md (BRANCH_LEN_P_128MiB): New constant.
        (BRANCH_LEN_N_128MiB): likewise.
        (BRANCH_LEN_P_1MiB): likewise.
        (BRANCH_LEN_N_1MiB): likewise.
        (BRANCH_LEN_P_32KiB): likewise.
        (BRANCH_LEN_N_32KiB): likewise.
        (BRANCH_LEN_P_1KiB): likewise.
        (BRANCH_LEN_N_1KiB): likewise.
---
 gcc/config/aarch64/aarch64.md | 68 ++++++++++++++++++++++++++---------
 1 file changed, 52 insertions(+), 16 deletions(-)

diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
index 23775ec58ca..ca5bd96a754 100644
--- a/gcc/config/aarch64/aarch64.md
+++ b/gcc/config/aarch64/aarch64.md
@@ -696,7 +696,27 @@ (define_insn "jump"
   [(set_attr "type" "branch")]
 )
 
+;; Maximum PC-relative positive/negative displacements for various branching
+;; instructions.
+(define_constants
+  [
+    ;; +/- 128MiB.  Used by B, BL.
+    (BRANCH_LEN_P_128Mib  134217724)
+    (BRANCH_LEN_N_128Mib -134217728)
+
+    ;; +/- 1MiB.  Used by B.<cond>, CBZ, CBNZ.
+    (BRANCH_LEN_P_1Mib  1048572)
+    (BRANCH_LEN_N_1Mib -1048576)
 
+    ;; +/- 32KiB.  Used by TBZ, TBNZ.
+    (BRANCH_LEN_P_32Kib  32764)
+    (BRANCH_LEN_N_32Kib -32768)
+
+    ;; +/- 1KiB.  Used by CBB<cond>, CBH<cond>, CB<cond>.
+    (BRANCH_LEN_P_1Kib  1020)
+    (BRANCH_LEN_N_1Kib -1024)
+  ]
+)
 
 ;; -------------------------------------------------------------------
 ;; Conditional jumps
@@ -760,13 +780,17 @@ (define_insn "aarch64_bcond"
   }
   [(set_attr "type" "branch")
    (set (attr "length")
-       (if_then_else (and (ge (minus (match_dup 2) (pc)) (const_int -1048576))
-                          (lt (minus (match_dup 2) (pc)) (const_int 1048572)))
+       (if_then_else (and (ge (minus (match_dup 2) (pc))
+                              (const_int BRANCH_LEN_N_1Mib))
+                          (lt (minus (match_dup 2) (pc))
+                              (const_int BRANCH_LEN_P_1Mib)))
                      (const_int 4)
                      (const_int 8)))
    (set (attr "far_branch")
-       (if_then_else (and (ge (minus (match_dup 2) (pc)) (const_int -1048576))
-                          (lt (minus (match_dup 2) (pc)) (const_int 1048572)))
+       (if_then_else (and (ge (minus (match_dup 2) (pc))
+                              (const_int BRANCH_LEN_N_1Mib))
+                          (lt (minus (match_dup 2) (pc))
+                              (const_int BRANCH_LEN_P_1Mib)))
                      (const_int 0)
                      (const_int 1)))]
 )
@@ -823,13 +847,17 @@ (define_insn "aarch64_cb<optab><mode>1"
   }
   [(set_attr "type" "branch")
    (set (attr "length")
-       (if_then_else (and (ge (minus (match_dup 1) (pc)) (const_int -1048576))
-                          (lt (minus (match_dup 1) (pc)) (const_int 1048572)))
+       (if_then_else (and (ge (minus (match_dup 1) (pc))
+                              (const_int BRANCH_LEN_N_1Mib))
+                          (lt (minus (match_dup 1) (pc))
+                              (const_int BRANCH_LEN_P_1Mib)))
                      (const_int 4)
                      (const_int 8)))
    (set (attr "far_branch")
-       (if_then_else (and (ge (minus (match_dup 2) (pc)) (const_int -1048576))
-                          (lt (minus (match_dup 2) (pc)) (const_int 1048572)))
+       (if_then_else (and (ge (minus (match_dup 2) (pc))
+                              (const_int BRANCH_LEN_N_1Mib))
+                          (lt (minus (match_dup 2) (pc))
+                              (const_int BRANCH_LEN_P_1Mib)))
                      (const_int 0)
                      (const_int 1)))]
 )
@@ -864,13 +892,17 @@ (define_insn "*cb<optab><mode>1"
   }
   [(set_attr "type" "branch")
    (set (attr "length")
-       (if_then_else (and (ge (minus (match_dup 1) (pc)) (const_int -32768))
-                          (lt (minus (match_dup 1) (pc)) (const_int 32764)))
+       (if_then_else (and (ge (minus (match_dup 1) (pc))
+                              (const_int BRANCH_LEN_N_32Kib))
+                          (lt (minus (match_dup 1) (pc))
+                              (const_int BRANCH_LEN_P_32Kib)))
                      (const_int 4)
                      (const_int 8)))
    (set (attr "far_branch")
-       (if_then_else (and (ge (minus (match_dup 1) (pc)) (const_int -1048576))
-                          (lt (minus (match_dup 1) (pc)) (const_int 1048572)))
+       (if_then_else (and (ge (minus (match_dup 1) (pc))
+                              (const_int BRANCH_LEN_N_1Mib))
+                          (lt (minus (match_dup 1) (pc))
+                              (const_int BRANCH_LEN_P_1Mib)))
                      (const_int 0)
                      (const_int 1)))]
 )
@@ -925,13 +957,17 @@ (define_insn "@aarch64_tb<optab><ALLI:mode><GPI:mode>"
   }
   [(set_attr "type" "branch")
    (set (attr "length")
-       (if_then_else (and (ge (minus (match_dup 2) (pc)) (const_int -32768))
-                          (lt (minus (match_dup 2) (pc)) (const_int 32764)))
+       (if_then_else (and (ge (minus (match_dup 2) (pc))
+                              (const_int BRANCH_LEN_N_32Kib))
+                          (lt (minus (match_dup 2) (pc))
+                              (const_int BRANCH_LEN_P_32Kib)))
                      (const_int 4)
                      (const_int 8)))
    (set (attr "far_branch")
-       (if_then_else (and (ge (minus (match_dup 2) (pc)) (const_int -1048576))
-                          (lt (minus (match_dup 2) (pc)) (const_int 1048572)))
+       (if_then_else (and (ge (minus (match_dup 2) (pc))
+                              (const_int BRANCH_LEN_N_1Mib))
+                          (lt (minus (match_dup 2) (pc))
+                              (const_int BRANCH_LEN_P_1Mib)))
                      (const_int 0)
                      (const_int 1)))]
 
-- 
2.45.2

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