Elf2: create a new linker from scratch

This iteration already has significantly better incremental support.

Closes #24110

Author: jacobly0

CMakeLists.txt

@@ -583,6 +583,7 @@ set(ZIG_STAGE2_SOURCES
     src/link/Elf/relocatable.zig
     src/link/Elf/relocation.zig
     src/link/Elf/synthetic_sections.zig
+    src/link/Elf2.zig
     src/link/Goff.zig
     src/link/LdScript.zig
     src/link/Lld.zig
@@ -612,6 +613,7 @@ set(ZIG_STAGE2_SOURCES
     src/link/MachO/synthetic.zig
     src/link/MachO/Thunk.zig
     src/link/MachO/uuid.zig
+    src/link/MappedFile.zig
     src/link/Queue.zig
     src/link/StringTable.zig
     src/link/Wasm.zig

build.zig

@@ -202,6 +202,7 @@ pub fn build(b: *std.Build) !void {
     });
     exe.pie = pie;
     exe.entitlements = entitlements;
+    exe.use_new_linker = b.option(bool, "new-linker", "Use the new linker");
 
     const use_llvm = b.option(bool, "use-llvm", "Use the llvm backend");
     exe.use_llvm = use_llvm;

ci/x86_64-linux-debug-llvm.sh

@@ -203,7 +203,7 @@ fn add_adjusted(a: f64, b: f64) f64 {
         if (uhii & 1 == 0) {
             // hibits += copysign(1.0, sum.hi, sum.lo)
             const uloi: u64 = @bitCast(sum.lo);
-            uhii += 1 - ((uhii ^ uloi) >> 62);
+            uhii = uhii + 1 - ((uhii ^ uloi) >> 62);
             sum.hi = @bitCast(uhii);
         }
     }
@@ -217,7 +217,7 @@ fn add_and_denorm(a: f64, b: f64, scale: i32) f64 {
         const bits_lost = -@as(i32, @intCast((uhii >> 52) & 0x7FF)) - scale + 1;
         if ((bits_lost != 1) == (uhii & 1 != 0)) {
             const uloi: u64 = @bitCast(sum.lo);
-            uhii += 1 - (((uhii ^ uloi) >> 62) & 2);
+            uhii = uhii + 1 - (((uhii ^ uloi) >> 62) & 2);
             sum.hi = @bitCast(uhii);
         }
     }
@@ -259,7 +259,7 @@ fn add_adjusted128(a: f128, b: f128) f128 {
         if (uhii & 1 == 0) {
             // hibits += copysign(1.0, sum.hi, sum.lo)
             const uloi: u128 = @bitCast(sum.lo);
-            uhii += 1 - ((uhii ^ uloi) >> 126);
+            uhii = uhii + 1 - ((uhii ^ uloi) >> 126);
             sum.hi = @bitCast(uhii);
         }
     }
@@ -284,7 +284,7 @@ fn add_and_denorm128(a: f128, b: f128, scale: i32) f128 {
         const bits_lost = -@as(i32, @intCast((uhii >> 112) & 0x7FFF)) - scale + 1;
         if ((bits_lost != 1) == (uhii & 1 != 0)) {
             const uloi: u128 = @bitCast(sum.lo);
-            uhii += 1 - (((uhii ^ uloi) >> 126) & 2);
+            uhii = uhii + 1 - (((uhii ^ uloi) >> 126) & 2);
             sum.hi = @bitCast(uhii);
         }
     }

lib/compiler_rt/fma.zig

@@ -192,6 +192,7 @@ want_lto: ?bool = null,
 
 use_llvm: ?bool,
 use_lld: ?bool,
+use_new_linker: ?bool,
 
 /// Corresponds to the `-fallow-so-scripts` / `-fno-allow-so-scripts` CLI
 /// flags, overriding the global user setting provided to the `zig build`
@@ -441,6 +442,7 @@ pub fn create(owner: *std.Build, options: Options) *Compile {
 
         .use_llvm = options.use_llvm,
         .use_lld = options.use_lld,
+        .use_new_linker = null,
 
         .zig_process = null,
     };
@@ -1096,6 +1098,7 @@ fn getZigArgs(compile: *Compile, fuzz: bool) ![][]const u8 {
 
     try addFlag(&zig_args, "llvm", compile.use_llvm);
     try addFlag(&zig_args, "lld", compile.use_lld);
+    try addFlag(&zig_args, "new-linker", compile.use_new_linker);
 
     if (compile.root_module.resolved_target.?.query.ofmt) |ofmt| {
         try zig_args.append(try std.fmt.allocPrint(arena, "-ofmt={s}", .{@tagName(ofmt)}));