Commit caf672c49586f1af5e3d41ae200aded991b8b0f7 | zig

author	Andrew Kelley <andrew@ziglang.org> 2019-02-09 21:10:59-05:00
committer	Andrew Kelley <andrew@ziglang.org> 2019-02-09 21:38:34-05:00
log	caf672c49586f1af5e3d41ae200aded991b8b0f7
tree	76b8e9dd955342c6a7ccda3ca62eade39edb4c5f
parent	31be1ddf09fafae270d9946a3f09aa25816dd153
signature	Commit is signed but in an unrecognized format.

`@truncate`: comptime 0 when target type is 0 bits

also if the dest type is a comptime_int, then treat it
as an implicit cast.

also compile error for attempting to truncate undefined

closes #1568

4 files changed, 56 insertions(+), 13 deletions(-)

doc
- langref.html.in
src
- ir.cpp
test
- compile_errors.zig
- stage1
  - behavior
    truncate.zig

doc/langref.html.in+7-4

...	@@ -6381,14 +6381,14 @@ fn List(comptime T: type) type {	...	@@ -6381,14 +6381,14 @@ fn List(comptime T: type) type {
6381	{#header_close#}	6381	{#header_close#}
6382		6382
6383	{#header_open\|@truncate#}	6383	{#header_open\|@truncate#}
6384	<pre>{#syntax#}@truncate(comptime T: type, integer) T{#endsyntax#}</pre>	6384	<pre>{#syntax#}@truncate(comptime T: type, integer: var) T{#endsyntax#}</pre>
6385	<p>	6385	<p>
6386	This function truncates bits from an integer type, resulting in a smaller	6386	This function truncates bits from an integer type, resulting in a smaller
6387	integer type.	6387	integer type.
6388	</p>	6388	</p>
6389	<p>	6389	<p>
6390	The following produces a crash in debug mode and undefined behavior in	6390	The following produces a crash in {#link\|Debug#} mode and {#link\|Undefined Behavior#} in
6391	release mode:	6391	{#link\|ReleaseFast#} mode:
6392	</p>	6392	</p>
6393	<pre>{#syntax#}const a: u16 = 0xabcd;	6393	<pre>{#syntax#}const a: u16 = 0xabcd;
6394	const b: u8 = u8(a);{#endsyntax#}</pre>	6394	const b: u8 = u8(a);{#endsyntax#}</pre>
...	@@ -6402,7 +6402,10 @@ const b: u8 = @truncate(u8, a);	...	@@ -6402,7 +6402,10 @@ const b: u8 = @truncate(u8, a);
6402	This function always truncates the significant bits of the integer, regardless	6402	This function always truncates the significant bits of the integer, regardless
6403	of endianness on the target platform.	6403	of endianness on the target platform.
6404	</p>	6404	</p>
6405		6405	<p>
		6406	If {#syntax#}T{#endsyntax#} is {#syntax#}comptime_int{#endsyntax#},
		6407	then this is semantically equivalent to an {#link\|implicit cast\|Implicit Casts#}.
		6408	</p>
6406	{#header_close#}	6409	{#header_close#}
6407		6410
6408	{#header_open\|@typeId#}	6411	{#header_open\|@typeId#}

src/ir.cpp+16-8

 @@ -18491,7 +18491,22 @@ static IrInstruction *ir_analyze_instruction_truncate(IrAnalyze *ira, IrInstruct
         return ira->codegen->invalid_instruction;
     }
-    if (src_type->data.integral.bit_count == 0) {
+    if (dest_type->id == ZigTypeIdComptimeInt) {
+        return ir_implicit_cast(ira, target, dest_type);
+    }
+    if (instr_is_comptime(target)) {
+        ConstExprValue *val = ir_resolve_const(ira, target, UndefBad);
+        if (val == nullptr)
+            return ira->codegen->invalid_instruction;
+        IrInstruction *result = ir_const(ira, &instruction->base, dest_type);
+        bigint_truncate(&result->value.data.x_bigint, &val->data.x_bigint,
+                dest_type->data.integral.bit_count, dest_type->data.integral.is_signed);
+        return result;
+    }
+    if (src_type->data.integral.bit_count == 0 || dest_type->data.integral.bit_count == 0) {
         IrInstruction *result = ir_const(ira, &instruction->base, dest_type);
         bigint_init_unsigned(&result->value.data.x_bigint, 0);
         return result;
 @@ -18507,13 +18522,6 @@ static IrInstruction *ir_analyze_instruction_truncate(IrAnalyze *ira, IrInstruct
         return ira->codegen->invalid_instruction;
     }
-    if (target->value.special == ConstValSpecialStatic) {
-        IrInstruction *result = ir_const(ira, &instruction->base, dest_type);
-        bigint_truncate(&result->value.data.x_bigint, &target->value.data.x_bigint,
-                dest_type->data.integral.bit_count, dest_type->data.integral.is_signed);
-        return result;
-    }
     IrInstruction *new_instruction = ir_build_truncate(&ira->new_irb, instruction->base.scope,
             instruction->base.source_node, dest_type_value, target);
     new_instruction->value.type = dest_type;

test/compile_errors.zig+10-1

 @@ -1,6 +1,15 @@
 const tests = @import("tests.zig");
 pub fn addCases(cases: *tests.CompileErrorContext) void {
+    cases.addTest(
+        "@truncate undefined value",
+        \\export fn entry() void {
+        \\    var z = @truncate(u8, u16(undefined));
+        \\}
+    ,
+        ".tmp_source.zig:2:30: error: use of undefined value",
+    );
     cases.addTest(
         "return invalid type from test",
         \\test "example" { return 1; }
 @@ -3335,7 +3344,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
     cases.add(
         "truncate sign mismatch",
         \\fn f() i8 {
-        \\    const x: u32 = 10;
+        \\    var x: u32 = 10;
         \\    return @truncate(i8, x);
         \\}
         \\

test/stage1/behavior/truncate.zig+23

...	@@ -6,3 +6,26 @@ test "truncate u0 to larger integer allowed and has comptime known result" {	...	@@ -6,3 +6,26 @@ test "truncate u0 to larger integer allowed and has comptime known result" {
6	const y = @truncate(u8, x);	6	const y = @truncate(u8, x);
7	comptime expect(y == 0);	7	comptime expect(y == 0);
8	}	8	}
		9
		10	test "truncate.u0.literal" {
		11	var z = @truncate(u0, 0);
		12	expect(z == 0);
		13	}
		14
		15	test "truncate.u0.const" {
		16	const c0: usize = 0;
		17	var z = @truncate(u0, c0);
		18	expect(z == 0);
		19	}
		20
		21	test "truncate.u0.var" {
		22	var d: u8 = 2;
		23	var z = @truncate(u0, d);
		24	expect(z == 0);
		25	}
		26
		27	test "truncate sign mismatch but comptime known so it works anyway" {
		28	const x: u32 = 10;
		29	var result = @truncate(i8, x);
		30	expect(result == 10);
		31	}