wasm: lower min/max for floats to compiler_rt

The min and max builtins in Zig have some intricate behavior
related to floats, that is not replicated with the min and max
wasm instructions or using simple select operations. By lowering
these instructions to compiler_rt, handling around NaNs is done
correctly.

See also WebAssembly/design#214

src/arch/wasm/CodeGen.zig

@@ -6234,8 +6234,10 @@ fn airMulWithOverflow(func: *CodeGen, inst: Air.Inst.Index) InnerError!void {
     func.finishAir(inst, result_ptr, &.{ extra.lhs, extra.rhs });
 }
 
-fn airMaxMin(func: *CodeGen, inst: Air.Inst.Index, op: enum { max, min }) InnerError!void {
+fn airMaxMin(func: *CodeGen, inst: Air.Inst.Index, op: Op) InnerError!void {
+    assert(op == .max or op == .min);
     const mod = func.bin_file.base.options.module.?;
+    const target = mod.getTarget();
     const bin_op = func.air.instructions.items(.data)[inst].bin_op;
 
     const ty = func.typeOfIndex(inst);
@@ -6250,13 +6252,25 @@ fn airMaxMin(func: *CodeGen, inst: Air.Inst.Index, op: enum { max, min }) InnerE
     const lhs = try func.resolveInst(bin_op.lhs);
     const rhs = try func.resolveInst(bin_op.rhs);
 
-    // operands to select from
-    try func.lowerToStack(lhs);
-    try func.lowerToStack(rhs);
-    _ = try func.cmp(lhs, rhs, ty, if (op == .max) .gt else .lt);
+    if (ty.zigTypeTag(mod) == .Float) {
+        var fn_name_buf: [64]u8 = undefined;
+        const float_bits = ty.floatBits(target);
+        const fn_name = std.fmt.bufPrint(&fn_name_buf, "{s}f{s}{s}", .{
+            target_util.libcFloatPrefix(float_bits),
+            @tagName(op),
+            target_util.libcFloatSuffix(float_bits),
+        }) catch unreachable;
+        const result = try func.callIntrinsic(fn_name, &.{ ty.ip_index, ty.ip_index }, ty, &.{ lhs, rhs });
+        try func.lowerToStack(result);
+    } else {
+        // operands to select from
+        try func.lowerToStack(lhs);
+        try func.lowerToStack(rhs);
+        _ = try func.cmp(lhs, rhs, ty, if (op == .max) .gt else .lt);
 
-    // based on the result from comparison, return operand 0 or 1.
-    try func.addTag(.select);
+        // based on the result from comparison, return operand 0 or 1.
+        try func.addTag(.select);
+    }
 
     // store result in local
     const result_ty = if (isByRef(ty, mod)) Type.u32 else ty;

test/behavior/maximum_minimum.zig

@@ -123,6 +123,12 @@ test "@min/max for floats" {
             try expectEqual(x, @min(y, x));
             try expectEqual(y, @max(x, y));
             try expectEqual(y, @max(y, x));
+
+            if (T != comptime_float) {
+                var nan: T = std.math.nan(T);
+                try expectEqual(y, @max(nan, y));
+                try expectEqual(y, @max(y, nan));
+            }
         }
     };