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Auto merge of rust-lang#120650 - clubby789:switchint-const, r=saethlin
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Use `br` instead of a conditional when switching on a constant boolean

r? `@ghost`
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bors committed Feb 23, 2024
2 parents 397937d + 15dad2b commit 77899b3
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Showing 2 changed files with 72 additions and 5 deletions.
18 changes: 13 additions & 5 deletions compiler/rustc_codegen_ssa/src/mir/block.rs
Original file line number Diff line number Diff line change
Expand Up @@ -319,7 +319,15 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
targets: &SwitchTargets,
) {
let discr = self.codegen_operand(bx, discr);
let discr_value = discr.immediate();
let switch_ty = discr.layout.ty;
// If our discriminant is a constant we can branch directly
if let Some(const_discr) = bx.const_to_opt_u128(discr_value, switch_ty.is_signed()) {
let target = targets.target_for_value(const_discr);
bx.br(helper.llbb_with_cleanup(self, target));
return;
};

let mut target_iter = targets.iter();
if target_iter.len() == 1 {
// If there are two targets (one conditional, one fallback), emit `br` instead of
Expand All @@ -330,14 +338,14 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
if switch_ty == bx.tcx().types.bool {
// Don't generate trivial icmps when switching on bool.
match test_value {
0 => bx.cond_br(discr.immediate(), llfalse, lltrue),
1 => bx.cond_br(discr.immediate(), lltrue, llfalse),
0 => bx.cond_br(discr_value, llfalse, lltrue),
1 => bx.cond_br(discr_value, lltrue, llfalse),
_ => bug!(),
}
} else {
let switch_llty = bx.immediate_backend_type(bx.layout_of(switch_ty));
let llval = bx.const_uint_big(switch_llty, test_value);
let cmp = bx.icmp(IntPredicate::IntEQ, discr.immediate(), llval);
let cmp = bx.icmp(IntPredicate::IntEQ, discr_value, llval);
bx.cond_br(cmp, lltrue, llfalse);
}
} else if self.cx.sess().opts.optimize == OptLevel::No
Expand All @@ -362,11 +370,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
let ll2 = helper.llbb_with_cleanup(self, target2);
let switch_llty = bx.immediate_backend_type(bx.layout_of(switch_ty));
let llval = bx.const_uint_big(switch_llty, test_value1);
let cmp = bx.icmp(IntPredicate::IntEQ, discr.immediate(), llval);
let cmp = bx.icmp(IntPredicate::IntEQ, discr_value, llval);
bx.cond_br(cmp, ll1, ll2);
} else {
bx.switch(
discr.immediate(),
discr_value,
helper.llbb_with_cleanup(self, targets.otherwise()),
target_iter.map(|(value, target)| (value, helper.llbb_with_cleanup(self, target))),
);
Expand Down
59 changes: 59 additions & 0 deletions tests/codegen/constant-branch.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,59 @@
//@ compile-flags: -Zmir-opt-level=0 -C no-prepopulate-passes -Copt-level=0
// make sure that branching on a constant does not emit a conditional
// branch or a switch

#![crate_type = "lib"]

// CHECK-LABEL: @if_bool
#[no_mangle]
pub fn if_bool() {
// CHECK: br label %{{.+}}
_ = if true {
0
} else {
1
};

// CHECK: br label %{{.+}}
_ = if false {
0
} else {
1
};
}

// CHECK-LABEL: @if_constant_int_eq
#[no_mangle]
pub fn if_constant_int_eq() {
let val = 0;
// CHECK: br label %{{.+}}
_ = if val == 0 {
0
} else {
1
};

// CHECK: br label %{{.+}}
_ = if val == 1 {
0
} else {
1
};
}

// CHECK-LABEL: @if_constant_match
#[no_mangle]
pub fn if_constant_match() {
// CHECK: br label %{{.+}}
_ = match 1 {
1 => 2,
2 => 3,
_ => 4
};

// CHECK: br label %{{.+}}
_ = match 1 {
2 => 3,
_ => 4
};
}

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