Limit CSE of integer constants (#10615)

Never do CSE for constants that fit in 32 bits signed.

Do it only for constants that do not fit, and only on some 64-bit targets
where the code generated for Iconst_int can be costly (ARM64, POWER, RISC-V).

Fixes: #10591

Changes

@@ -22,6 +22,11 @@ Working version
   parameter passing on PowerPC (16 registers) and on s390x (8 registers).
   (Xavier Leroy, review by Mark Shinwell)
 
+- #10591, #10615: Tune the heuristic for CSE of integer constants
+  so as to avoid excessive CSE on compiler-generated constants
+  and long register allocation times.
+  (Xavier Leroy, report by Edwin Török, review by Nicolás Ojeda Bär)
+
 ### Standard library:
 
 * #7812, #10475: `Filename.chop_suffix name suff` now checks that `suff`

asmcomp/arm/CSE.ml

@@ -29,11 +29,6 @@ method! class_of_operation op =
   | Ispecific _ -> Op_pure
   | _ -> super#class_of_operation op
 
-method! is_cheap_operation op =
-  match op with
-  | Iconst_int n -> n <= 255n && n >= 0n
-  | _ -> false
-
 end
 
 let fundecl f =

asmcomp/arm64/CSE.ml

@@ -31,7 +31,7 @@ method! class_of_operation op =
 
 method! is_cheap_operation op =
   match op with
-  | Iconst_int n -> n <= 65535n && n >= 0n
+  | Iconst_int n -> n <= 0x7FFF_FFFFn && n >= -0x8000_0000n
   | _ -> false
 
 end

asmcomp/power/CSE.ml

@@ -31,7 +31,7 @@ method! class_of_operation op =
 
 method! is_cheap_operation op =
   match op with
-  | Iconst_int n -> n <= 32767n && n >= -32768n
+  | Iconst_int n -> n <= 0x7FFF_FFFFn && n >= -0x8000_0000n
   | _ -> false
 
 end

asmcomp/riscv/CSE.ml

@@ -30,7 +30,7 @@ method! class_of_operation op =
 
 method! is_cheap_operation op =
   match op with
-  | Iconst_int n -> n <= 0x7FFn && n >= -0x800n
+  | Iconst_int n -> n <= 0x7FFF_FFFFn && n >= -0x8000_0000n
   | _ -> false
 
 end