[clang] [llvm] [clang][CodeGen][AA] Introduce `!llvm.errno.tbaa` for errno alias disambiguation (PR #125258)

[Nikita Popov via cfe-commits]

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@@ -0,0 +1,90 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=instcombine < %s | FileCheck %s
+
+; sinf clobbering errno, but %p cannot alias errno per C/C++ strict aliasing rules via TBAA.
+; Hence, can do constant store-to-load forwarding.
+define float @does_not_alias_errno(ptr noundef %p, float noundef %f) {
+; CHECK-LABEL: define float @does_not_alias_errno(
+; CHECK-SAME: ptr noundef [[P:%.*]], float noundef [[F:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    store float 0.000000e+00, ptr [[P]], align 4, !tbaa [[TBAA4:![0-9]+]]
+; CHECK-NEXT:    [[CALL:%.*]] = call float @sinf(float noundef [[F]])
+; CHECK-NEXT:    ret float 0.000000e+00
+;
+entry:
+  store float 0.000000e+00, ptr %p, align 4, !tbaa !4
+  %call = call float @sinf(float noundef %f)
+  %0 = load float, ptr %p, align 4, !tbaa !4
+  ret float %0
+}
+
+; sinf clobbering errno, but %p is alloca memory, wich can never aliases errno.
+; Hence, can do constant store-to-load forwarding.
+define float @does_not_alias_errno_2(float %f) {
+; CHECK-LABEL: define float @does_not_alias_errno_2(
+; CHECK-SAME: float [[F:%.*]]) {
+; CHECK-NEXT:    [[P:%.*]] = alloca float, align 4
+; CHECK-NEXT:    call void @escape(ptr nonnull [[P]])
+; CHECK-NEXT:    store float 0.000000e+00, ptr [[P]], align 4
+; CHECK-NEXT:    [[TMP1:%.*]] = call float @sinf(float [[F]])
+; CHECK-NEXT:    ret float 0.000000e+00
+;
+  %p = alloca float
+  call void @escape(ptr %p)
+  store float 0.0, ptr %p
+  call float @sinf(float %f)
+  %v = load float, ptr %p
+  ret float %v
+}
+
+; sinf clobbering errno, but %p is memory accessed w/ size larger than errno.
+; Hence, can do constant store-to-load forwarding.
+define double @does_not_alias_errno_3(ptr %p, float %f) {
+; CHECK-LABEL: define double @does_not_alias_errno_3(
+; CHECK-SAME: ptr [[P:%.*]], float [[F:%.*]]) {
+; CHECK-NEXT:    call void @escape(ptr [[P]])
+; CHECK-NEXT:    store double 0.000000e+00, ptr [[P]], align 8
+; CHECK-NEXT:    [[TMP1:%.*]] = call float @sinf(float [[F]])
+; CHECK-NEXT:    ret double 0.000000e+00
+;
+  call void @escape(ptr %p)
+  store double 0.0, ptr %p
+  call float @sinf(float %f)
+  %v = load double, ptr %p
+  ret double %v
+}
+
+; sinf clobbering errno, unknown TBAA info, %p may alias errno.
+define float @may_alias_errno(ptr noundef %p, float noundef %f) {
+; CHECK-LABEL: define float @may_alias_errno(
+; CHECK-SAME: ptr noundef [[P:%.*]], float noundef [[F:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    store float 0.000000e+00, ptr [[P]], align 4
+; CHECK-NEXT:    [[CALL:%.*]] = call float @sinf(float noundef [[F]])
+; CHECK-NEXT:    [[TMP0:%.*]] = load float, ptr [[P]], align 4
+; CHECK-NEXT:    ret float [[TMP0]]
+;
+entry:
+  store float 0.000000e+00, ptr %p, align 4
+  %call = call float @sinf(float noundef %f)
+  %0 = load float, ptr %p, align 4
+  ret float %0
+}
+
+declare float @sinf(float noundef) memory(errnomem: write)
+declare void @escape(ptr %p)
+
+!llvm.errno.tbaa = !{!0}
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nikic wrote:

We should add Verifier checks that this metadata is well-formed.

We should also test that the multi-operand form works correctly.

https://github.com/llvm/llvm-project/pull/125258