Multiple stores generated for writes to volatile packed struct(u32)

[Lambosaurus]

Zig Version

0.11.0

Steps to Reproduce and Observed Behavior

When compiling with -O ReleaseSmall -target thumb-freestanding-none, strange stores are generated when writing to a volatile packed struct. This behavior seems to be related to -fstrip somehow, because the expected output is correctly generated with -fno-strip or -O ReleaseFast

The output can be seen in godbolt here.

Given the following source:

pub const Peripheral = packed struct {
    register: packed struct(u32) {
        bit0: u1 = 0,
        bit1: u1 = 0,
        bit2: u1 = 0,
        bit3: u1 = 0,
        bit4: u1 = 0,
        bit5: u2 = 0,
        bit7: u1 = 0,
        bit8: u24 = 0,
    }
};

pub const peripheral: *volatile Peripheral = @ptrFromInt(0x40021000);

export fn store_by_struct() void {
    peripheral.register = .{ .bit1 = 1 };
}

The resulting assembly is:

store_by_struct:
        ldr     r0, .LCPI0_0
        ldr     r1, [r0]
        orr     r1, r1, #2
        str     r1, [r0]
        ldr     r1, [r0]
        bic     r1, r1, #1
        str     r1, [r0]
        ldr     r1, [r0]
        bic     r1, r1, #4
        str     r1, [r0]
        ldr     r1, [r0]
        bic     r1, r1, #8
        str     r1, [r0]
        ldr     r1, [r0]
        bic     r1, r1, #16
        str     r1, [r0]
        ldr     r1, [r0]
        bic     r1, r1, #96
        str     r1, [r0]
        ldr     r1, [r0]
        bic     r1, r1, #128
        str     r1, [r0]
        ldr.w   r1, [r0, #1]
        uxtb    r1, r1
        str.w   r1, [r0, #1]
        bx      lr
.LCPI0_0:
        .long   1073876992

This seems to be generating a bit clear and a store for each field in the struct.

Expected Behavior

When compiling with -fno-strip the following assembly is generated:

store_by_struct:
        ldr     r0, .LCPI0_0
        movs    r1, #2
        str     r1, [r0]
        bx      lr
.LCPI0_0:
        .long   1073876992

This is the expected output.

[cfillion]

I'm running into a similar issue, however here with 0.11.0 neither -fstrip or -O ReleaseSmall were required to duplicate. The bad assembly was generated even in Debug or ReleaseFast builds.

Starting with commit 8b91611 onwards the issue is only triggered when -fstrip or -O ReleaseSmall is specified.

var value: u32 = 42;

const register: *volatile packed union {
  fields: packed struct(u32) {
    foo: bool = false,
    _1:  u3   = 0,
    _2:  u4   = 0,
    bar: bool = false,
    _3:  u3   = 0,
    baz: bool = false,
    _6:  u19  = 0,
  },
  value: u32,
} = @ptrCast(&value);

export fn _start() void {
  register.fields = .{ .foo = true, .bar = true };
}

$ git switch -d 8b916117~1
HEAD is now at b835fd90ce std.http.Server: use correct header for Transfer-Encoding
$ rm -r build zig-cache t t.o; cmake -B build -G Ninja && cmake --build build
$ build/stage3/bin/zig build-exe t.zig -target thumb-freestanding && arm-none-eabi-objdump -d t
00020104 <_start>:
   20104:	f240 114c 	movw	r1, #332	@ 0x14c
   20108:	f2c0 0103 	movt	r1, #3
   2010c:	6808      	ldr	r0, [r1, #0]
   2010e:	f040 0001 	orr.w	r0, r0, #1
   20112:	6008      	str	r0, [r1, #0]
   20114:	f8d1 0001 	ldr.w	r0, [r1, #1]
   20118:	f440 7080 	orr.w	r0, r0, #256	@ 0x100
   2011c:	f8c1 0001 	str.w	r0, [r1, #1]
   20120:	6808      	ldr	r0, [r1, #0]
   20122:	f020 000e 	bic.w	r0, r0, #14
   20126:	6008      	str	r0, [r1, #0]
   20128:	6808      	ldr	r0, [r1, #0]
   2012a:	f020 00f0 	bic.w	r0, r0, #240	@ 0xf0
   2012e:	6008      	str	r0, [r1, #0]
   20130:	6808      	ldr	r0, [r1, #0]
   20132:	f420 6060 	bic.w	r0, r0, #3584	@ 0xe00
   20136:	6008      	str	r0, [r1, #0]
   20138:	6808      	ldr	r0, [r1, #0]
   2013a:	f420 5080 	bic.w	r0, r0, #4096	@ 0x1000
   2013e:	6008      	str	r0, [r1, #0]
   20140:	6808      	ldr	r0, [r1, #0]
   20142:	f36f 305f 	bfc	r0, #13, #19
   20146:	6008      	str	r0, [r1, #0]
   20148:	4770      	bx	lr

$ git switch -d 8b916117
Previous HEAD position was b835fd90ce std.http.Server: use correct header for Transfer-Encoding
HEAD is now at 8b9161179d Sema: avoid deleting runtime side-effects in comptime initializers
$ rm -r build zig-cache t t.o; cmake -B build -G Ninja && cmake --build build
$ build/stage3/bin/zig build-exe t.zig -target thumb-freestanding && arm-none-eabi-objdump -d t
00020104 <_start>:
   20104:	f240 1114 	movw	r1, #276	@ 0x114
   20108:	f2c0 0103 	movt	r1, #3
   2010c:	f240 1001 	movw	r0, #257	@ 0x101
   20110:	6008      	str	r0, [r1, #0]
   20112:	4770      	bx	lr

Adding -fstrip reverts the output back to how it was before that commit.

Using a temporary variable does not trigger the issue:

export fn _start() void {
  const fields = .{ .foo = true, .bar = true };
  register.fields = fields;
}