Stream: git-wasmtime

Topic: wasmtime / issue #1598 Add volatile flag to MemFlags

Wasmtime GitHub notifications bot (May 04 2022 at 20:44):

cfallin labeled issue #1598:

Add Volatile flag to MemFlags.

Benefit

When some optimization pass knows about load/store type (is it volatile or not?) then it's possible to do a lot of optimizations. One of it is memory to register promotion. I tried to implement mem2reg pass there cranelift-mem2reg and this pass works but in some cases it fails e.g this simple C code:

// compiled with rcc
typedef struct Foo {
    int x;
    int y;
} Foo;

int main() {
    Foo foo;
    foo.y = 42;
}

Message:  definition error: Compilation error: Verifier errors
note: while compiling function u0:0() -> i32 system_v {
    ss0 = explicit_slot 8

block0:
    v1 = iconst.i64 4
    v2 = iadd.i64 v0, v1
    v3 = iconst.i32 42
    store v3, v2
    v4 = iconst.i32 0
    return v4
}

All loads and stores might be marked as volatile and mem2reg will not optimize struct Foo into registers.
Volatile flag also can help in SRoA when it's possible to replace allocation on stack with single 32 or 64 bit register.

Implementation

Just add additional enum case in codegen/ir/memflags.rs and add 3 simple functions to MemFlags: volatile() -> Self,is_volatile() -> bool, set_volatile(&mut self).

Alternatives

I do not see any alternatives.

UPD: This might be useful for this issue: Support volatile store/loads

Wasmtime GitHub notifications bot (May 04 2022 at 20:45):

cfallin labeled issue #1598:

Add Volatile flag to MemFlags.

Benefit

When some optimization pass knows about load/store type (is it volatile or not?) then it's possible to do a lot of optimizations. One of it is memory to register promotion. I tried to implement mem2reg pass there cranelift-mem2reg and this pass works but in some cases it fails e.g this simple C code:

// compiled with rcc
typedef struct Foo {
    int x;
    int y;
} Foo;

int main() {
    Foo foo;
    foo.y = 42;
}

Message:  definition error: Compilation error: Verifier errors
note: while compiling function u0:0() -> i32 system_v {
    ss0 = explicit_slot 8

block0:
    v1 = iconst.i64 4
    v2 = iadd.i64 v0, v1
    v3 = iconst.i32 42
    store v3, v2
    v4 = iconst.i32 0
    return v4
}

All loads and stores might be marked as volatile and mem2reg will not optimize struct Foo into registers.
Volatile flag also can help in SRoA when it's possible to replace allocation on stack with single 32 or 64 bit register.

Implementation

Just add additional enum case in codegen/ir/memflags.rs and add 3 simple functions to MemFlags: volatile() -> Self,is_volatile() -> bool, set_volatile(&mut self).

Alternatives

I do not see any alternatives.

UPD: This might be useful for this issue: Support volatile store/loads

Last updated: Dec 23 2024 at 12:05 UTC