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stage2: improve @sizeOf and @alignOf integers

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Prior to this commit, the logic for ABI size and ABI alignment for
integers was naive and incorrect. This results in wasted hardware as
well as undefined behavior in the LLVM backend when we memset an
incorrect number of bytes to 0xaa due to disagreeing with LLVM about the
ABI size of integers.

This commit introduces a "max int align" value which is different per
Target. This value is used to derive the ABI size and alignment of all
integers.

This commit makes an interesting change from stage1, which treats
128-bit integers as 16-bytes aligned for x86_64-linux. stage1 is
incorrect. The maximum integer alignment on this system is only 8 bytes.
This change breaks the behavior test called "128-bit cmpxchg" because on
that target, 128-bit cmpxchg does require a 16-bytes aligned pointer to
a 128 bit integer. However, this alignment property does not belong on
*all* 128 bit integers - only on the pointer type in the `@cmpxchg`
builtin function prototype. The user can then use an alignment override
annotation on a 128-bit integer variable or struct field to obtain such
a pointer.
This commit is contained in:
Andrew Kelley 2022-05-03 23:44:32 -07:00
parent 080e870a71
commit 259f784241
6 changed files with 211 additions and 60 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

77
lib/std/target.zig
View File

@ -1773,6 +1773,83 @@ pub const Target = struct {
else => false,
};
}
pub inline fn maxIntAlignment(target: Target) u16 {
return switch (target.cpu.arch) {
.avr => 1,
.msp430 => 2,
.xcore => 4,
.arm,
.armeb,
.thumb,
.thumbeb,
.x86_64,
.hexagon,
.mips,
.mipsel,
.mips64,
.mips64el,
.powerpc,
.powerpcle,
.powerpc64,
.powerpc64le,
.r600,
.amdgcn,
.riscv32,
.riscv64,
.sparc,
.sparcv9,
.sparcel,
.s390x,
.lanai,
.wasm32,
.wasm64,
=> 8,
.i386 => return switch (target.os.tag) {
.windows => 8,
else => 4,
},
.aarch64,
.aarch64_be,
.aarch64_32,
.bpfel,
.bpfeb,
.nvptx,
.nvptx64,
=> 16,
// Below this comment are unverified and I have chosen a number
// based on ptrBitWidth.
.spu_2 => 2,
.csky,
.arc,
.m68k,
.tce,
.tcele,
.le32,
.amdil,
.hsail,
.spir,
.kalimba,
.renderscript32,
.spirv32,
.shave,
=> 4,
.le64,
.amdil64,
.hsail64,
.spir64,
.renderscript64,
.ve,
.spirv64,
=> 8,
};
}
};
test {

2
src/codegen/llvm.zig
View File

@ -7583,7 +7583,7 @@ pub const FuncGen = struct {
const size_bytes = elem_ty.abiSize(target);
_ = self.builder.buildMemCpy(
self.builder.buildBitCast(ptr, llvm_ptr_u8, ""),
ptr_ty.ptrAlignment(target),
ptr_alignment,
self.builder.buildBitCast(elem, llvm_ptr_u8, ""),
elem_ty.abiAlignment(target),
self.context.intType(Type.usize.intInfo(target).bits).constInt(size_bytes, .False),

38
src/type.zig
View File

@ -2788,11 +2788,6 @@ pub const Type = extern union {
return AbiAlignmentAdvanced{ .scalar = target_util.defaultFunctionAlignment(target) };
},
.i16, .u16 => return AbiAlignmentAdvanced{ .scalar = 2 },
.i32, .u32 => return AbiAlignmentAdvanced{ .scalar = 4 },
.i64, .u64 => return AbiAlignmentAdvanced{ .scalar = 8 },
.u128, .i128 => return AbiAlignmentAdvanced{ .scalar = 16 },
.isize,
.usize,
.single_const_pointer_to_comptime_int,
@ -2865,14 +2860,15 @@ pub const Type = extern union {
// ABI alignment of vectors?
.vector => return AbiAlignmentAdvanced{ .scalar = 16 },
.i16, .u16 => return AbiAlignmentAdvanced{ .scalar = intAbiAlignment(16, target) },
.i32, .u32 => return AbiAlignmentAdvanced{ .scalar = intAbiAlignment(32, target) },
.i64, .u64 => return AbiAlignmentAdvanced{ .scalar = intAbiAlignment(64, target) },
.u128, .i128 => return AbiAlignmentAdvanced{ .scalar = intAbiAlignment(128, target) },
.int_signed, .int_unsigned => {
const bits: u16 = ty.cast(Payload.Bits).?.data;
if (bits == 0) return AbiAlignmentAdvanced{ .scalar = 0 };
if (bits <= 8) return AbiAlignmentAdvanced{ .scalar = 1 };
if (bits <= 16) return AbiAlignmentAdvanced{ .scalar = 2 };
if (bits <= 32) return AbiAlignmentAdvanced{ .scalar = 4 };
if (bits <= 64) return AbiAlignmentAdvanced{ .scalar = 8 };
return AbiAlignmentAdvanced{ .scalar = 16 };
return AbiAlignmentAdvanced{ .scalar = intAbiAlignment(bits, target) };
},
.optional => {
@ -3113,10 +3109,6 @@ pub const Type = extern union {
assert(elem_size >= payload.elem_type.abiAlignment(target));
return (payload.len + 1) * elem_size;
},
.i16, .u16 => return 2,
.i32, .u32 => return 4,
.i64, .u64 => return 8,
.u128, .i128 => return 16,
.isize,
.usize,
@ -3189,10 +3181,14 @@ pub const Type = extern union {
.error_set_merged,
=> return 2, // TODO revisit this when we have the concept of the error tag type
.i16, .u16 => return intAbiSize(16, target),
.i32, .u32 => return intAbiSize(32, target),
.i64, .u64 => return intAbiSize(64, target),
.u128, .i128 => return intAbiSize(128, target),
.int_signed, .int_unsigned => {
const bits: u16 = self.cast(Payload.Bits).?.data;
if (bits == 0) return 0;
return std.math.ceilPowerOfTwoPromote(u16, (bits + 7) / 8);
return intAbiSize(bits, target);
},
.optional => {
@ -3234,6 +3230,18 @@ pub const Type = extern union {
};
}
fn intAbiSize(bits: u16, target: Target) u64 {
const alignment = intAbiAlignment(bits, target);
return std.mem.alignForwardGeneric(u64, (bits + 7) / 8, alignment);
}
fn intAbiAlignment(bits: u16, target: Target) u32 {
return @minimum(
std.math.ceilPowerOfTwoPromote(u16, (bits + 7) / 8),
target.maxIntAlignment(),
);
}
/// Asserts the type has the bit size already resolved.
pub fn bitSize(ty: Type, target: Target) u64 {
return switch (ty.tag()) {

130
test/behavior/align.zig
View File

@ -47,41 +47,121 @@ fn expects4(x: *align(4) u32) void {
x.* += 1;
}
test "alignment of structs" {
test "alignment of struct with pointer has same alignment as usize" {
try expect(@alignOf(struct {
a: i32,
b: *i32,
}) == @alignOf(usize));
}
test "alignment of >= 128-bit integer type" {
try expect(@alignOf(u128) == 16);
try expect(@alignOf(u129) == 16);
}
test "alignment of struct with 128-bit field" {
try expect(@alignOf(struct {
test "alignment and size of structs with 128-bit fields" {
const A = struct {
x: u128,
}) == 16);
comptime {
try expect(@alignOf(struct {
x: u128,
}) == 16);
}
}
test "size of extern struct with 128-bit field" {
try expect(@sizeOf(extern struct {
};
const B = extern struct {
x: u128,
y: u8,
}) == 32);
};
const expected = switch (builtin.cpu.arch) {
.arm,
.armeb,
.thumb,
.thumbeb,
.x86_64,
.hexagon,
.mips,
.mipsel,
.mips64,
.mips64el,
.powerpc,
.powerpcle,
.powerpc64,
.powerpc64le,
.r600,
.amdgcn,
.riscv32,
.riscv64,
.sparc,
.sparcv9,
.sparcel,
.s390x,
.lanai,
.wasm32,
.wasm64,
=> .{
.a_align = 8,
.a_size = 16,
.b_align = 8,
.b_size = 24,
.u128_align = 8,
.u128_size = 16,
.u129_align = 8,
.u129_size = 24,
},
.i386 => switch (builtin.os.tag) {
.windows => .{
.a_align = 8,
.a_size = 16,
.b_align = 8,
.b_size = 24,
.u128_align = 8,
.u128_size = 16,
.u129_align = 8,
.u129_size = 24,
},
else => .{
.a_align = 4,
.a_size = 16,
.b_align = 4,
.b_size = 20,
.u128_align = 4,
.u128_size = 16,
.u129_align = 4,
.u129_size = 20,
},
},
.aarch64,
.aarch64_be,
.aarch64_32,
.bpfel,
.bpfeb,
.nvptx,
.nvptx64,
=> .{
.a_align = 16,
.a_size = 16,
.b_align = 16,
.b_size = 32,
.u128_align = 16,
.u128_size = 16,
.u129_align = 16,
.u129_size = 32,
},
else => return error.SkipZigTest,
};
comptime {
try expect(@sizeOf(extern struct {
x: u128,
y: u8,
}) == 32);
std.debug.assert(@alignOf(A) == expected.a_align);
std.debug.assert(@sizeOf(A) == expected.a_size);
std.debug.assert(@alignOf(B) == expected.b_align);
std.debug.assert(@sizeOf(B) == expected.b_size);
std.debug.assert(@alignOf(u128) == expected.u128_align);
std.debug.assert(@sizeOf(u128) == expected.u128_size);
std.debug.assert(@alignOf(u129) == expected.u129_align);
std.debug.assert(@sizeOf(u129) == expected.u129_size);
}
}
@ -328,7 +408,6 @@ test "read 128-bit field from default aligned struct in stack memory" {
.nevermind = 1,
.badguy = 12,
};
try expect((@ptrToInt(&default_aligned.badguy) % 16) == 0);
try expect(12 == default_aligned.badguy);
}
@ -345,7 +424,6 @@ test "read 128-bit field from default aligned struct in global memory" {
if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
try expect((@ptrToInt(&default_aligned_global.badguy) % 16) == 0);
try expect(12 == default_aligned_global.badguy);
}

15
test/behavior/bitcast.zig
View File

@ -138,18 +138,9 @@ test "@bitCast packed structs at runtime and comptime" {
fn doTheTest() !void {
var full = Full{ .number = 0x1234 };
var two_halves = @bitCast(Divided, full);
switch (native_endian) {
.Big => {
try expect(two_halves.half1 == 0x12);
try expect(two_halves.quarter3 == 0x3);
try expect(two_halves.quarter4 == 0x4);
},
.Little => {
try expect(two_halves.half1 == 0x34);
try expect(two_halves.quarter3 == 0x2);
try expect(two_halves.quarter4 == 0x1);
},
}
try expect(two_halves.half1 == 0x34);
try expect(two_halves.quarter3 == 0x2);
try expect(two_halves.quarter4 == 0x1);
}
};
try S.doTheTest();

9
test/behavior/struct.zig
View File

@ -499,17 +499,14 @@ const Bitfields = packed struct {
f7: u8,
};
test "native bit field understands endianness" {
if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
test "packed struct fields are ordered from LSB to MSB" {
if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
if (builtin.zig_backend == .stage2_c) return error.SkipZigTest; // TODO
if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
var all: u64 = if (native_endian != .Little)
0x1111222233445677
else
0x7765443322221111;
var all: u64 = 0x7765443322221111;
var bytes: [8]u8 = undefined;
@memcpy(&bytes, @ptrCast([*]u8, &all), 8);
var bitfields = @ptrCast(*Bitfields, &bytes).*;