bf5d96c96c
Add the `dead_on_unwind` attribute, which states that the caller will
not read from this argument if the call unwinds. This allows eliding
stores that could otherwise be visible on the unwind path, for example:
```
declare void @may_unwind()
define void @src(ptr noalias dead_on_unwind %out) {
store i32 0, ptr %out
call void @may_unwind()
store i32 1, ptr %out
ret void
}
define void @tgt(ptr noalias dead_on_unwind %out) {
call void @may_unwind()
store i32 1, ptr %out
ret void
}
```
The optimization is not valid without `dead_on_unwind`, because the `i32
0` value might be read if `@may_unwind` unwinds.
This attribute is primarily intended to be used on sret arguments. In
fact, I previously wanted to change the semantics of sret to include
this "no read after unwind" property (see D116998), but based on the
feedback there it is better to keep these attributes orthogonal (sret is
an ABI attribute, dead_on_unwind is an optimization attribute). This is
a reboot of that change with a separate attribute.
106 lines
3.4 KiB
LLVM
106 lines
3.4 KiB
LLVM
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
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; RUN: opt -S -passes=memcpyopt < %s -verify-memoryssa | FileCheck %s
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declare void @may_throw(ptr nocapture %x)
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define void @test1(ptr nocapture noalias dereferenceable(4) %x) {
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; CHECK-LABEL: @test1(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[T:%.*]] = alloca i32, align 4
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; CHECK-NEXT: call void @may_throw(ptr nonnull [[T]])
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; CHECK-NEXT: [[LOAD:%.*]] = load i32, ptr [[T]], align 4
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; CHECK-NEXT: store i32 [[LOAD]], ptr [[X:%.*]], align 4
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; CHECK-NEXT: ret void
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;
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entry:
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%t = alloca i32, align 4
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call void @may_throw(ptr nonnull %t)
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%load = load i32, ptr %t, align 4
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store i32 %load, ptr %x, align 4
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ret void
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}
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declare void @always_throws()
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define void @test2(ptr nocapture noalias dereferenceable(4) %x) {
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; CHECK-LABEL: @test2(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[T:%.*]] = alloca i32, align 4
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; CHECK-NEXT: call void @may_throw(ptr nonnull [[T]]) #[[ATTR0:[0-9]+]]
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; CHECK-NEXT: [[LOAD:%.*]] = load i32, ptr [[T]], align 4
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; CHECK-NEXT: call void @always_throws()
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; CHECK-NEXT: store i32 [[LOAD]], ptr [[X:%.*]], align 4
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; CHECK-NEXT: ret void
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;
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entry:
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%t = alloca i32, align 4
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call void @may_throw(ptr nonnull %t) nounwind
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%load = load i32, ptr %t, align 4
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call void @always_throws()
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store i32 %load, ptr %x, align 4
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ret void
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}
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; byval argument is not visible on unwind.
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define void @test_byval(ptr nocapture noalias dereferenceable(4) byval(i32) %x) {
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; CHECK-LABEL: @test_byval(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[T:%.*]] = alloca i32, align 4
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; CHECK-NEXT: call void @may_throw(ptr nonnull [[X:%.*]])
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; CHECK-NEXT: ret void
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;
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entry:
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%t = alloca i32, align 4
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call void @may_throw(ptr nonnull %t)
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%load = load i32, ptr %t, align 4
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store i32 %load, ptr %x, align 4
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ret void
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}
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define void @test_dead_on_unwind(ptr nocapture noalias writable dead_on_unwind dereferenceable(4) %x) {
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; CHECK-LABEL: @test_dead_on_unwind(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[T:%.*]] = alloca i32, align 4
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; CHECK-NEXT: call void @may_throw(ptr nonnull [[X:%.*]])
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; CHECK-NEXT: ret void
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;
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entry:
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%t = alloca i32, align 4
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call void @may_throw(ptr nonnull %t)
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%load = load i32, ptr %t, align 4
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store i32 %load, ptr %x, align 4
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ret void
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}
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; Same as previous test, but dereferenceability information is provided by sret.
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define void @test_dead_on_unwind_sret(ptr nocapture noalias writable dead_on_unwind sret(i32) %x) {
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; CHECK-LABEL: @test_dead_on_unwind_sret(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[T:%.*]] = alloca i32, align 4
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; CHECK-NEXT: call void @may_throw(ptr nonnull [[X:%.*]])
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; CHECK-NEXT: ret void
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;
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entry:
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%t = alloca i32, align 4
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call void @may_throw(ptr nonnull %t)
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%load = load i32, ptr %t, align 4
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store i32 %load, ptr %x, align 4
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ret void
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}
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define void @test_dead_on_unwind_missing_writable(ptr nocapture noalias dead_on_unwind dereferenceable(4) %x) {
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; CHECK-LABEL: @test_dead_on_unwind_missing_writable(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[T:%.*]] = alloca i32, align 4
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; CHECK-NEXT: call void @may_throw(ptr nonnull [[T]])
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; CHECK-NEXT: [[LOAD:%.*]] = load i32, ptr [[T]], align 4
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; CHECK-NEXT: store i32 [[LOAD]], ptr [[X:%.*]], align 4
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; CHECK-NEXT: ret void
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;
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entry:
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%t = alloca i32, align 4
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call void @may_throw(ptr nonnull %t)
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%load = load i32, ptr %t, align 4
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store i32 %load, ptr %x, align 4
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ret void
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}
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