From 673dbe1b5eef09db39783c828a84f1213a47bad0 Mon Sep 17 00:00:00 2001
From: Eli Friedman <efriedma@quicinc.com>
Date: Thu, 30 Jul 2020 17:32:39 -0700
Subject: [PATCH] [clang codegen] Use IR "align" attribute for static array
 arguments.

Without the "align" attribute, marking the argument dereferenceable is
basically useless.  See also D80166.

Fixes https://bugs.llvm.org/show_bug.cgi?id=46876 .

Differential Revision: https://reviews.llvm.org/D84992
---
 clang/lib/CodeGen/CGCall.cpp | 16 ++++++++++++----
 clang/test/CodeGen/vla.c     | 10 +++++-----
 2 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index 9d225b23e3c3..98ba1efc20de 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -2520,6 +2520,9 @@ void CodeGenFunction::EmitFunctionProlog(const CGFunctionInfo &FI,
             // bytes).
             if (ArrTy->getSizeModifier() == ArrayType::Static) {
               QualType ETy = ArrTy->getElementType();
+              llvm::Align Alignment =
+                  CGM.getNaturalTypeAlignment(ETy).getAsAlign();
+              AI->addAttrs(llvm::AttrBuilder().addAlignmentAttr(Alignment));
               uint64_t ArrSize = ArrTy->getSize().getZExtValue();
               if (!ETy->isIncompleteType() && ETy->isConstantSizeType() &&
                   ArrSize) {
@@ -2539,10 +2542,15 @@ void CodeGenFunction::EmitFunctionProlog(const CGFunctionInfo &FI,
             // For C99 VLAs with the static keyword, we don't know the size so
             // we can't use the dereferenceable attribute, but in addrspace(0)
             // we know that it must be nonnull.
-            if (ArrTy->getSizeModifier() == VariableArrayType::Static &&
-                !getContext().getTargetAddressSpace(ArrTy->getElementType()) &&
-                !CGM.getCodeGenOpts().NullPointerIsValid)
-              AI->addAttr(llvm::Attribute::NonNull);
+            if (ArrTy->getSizeModifier() == VariableArrayType::Static) {
+              QualType ETy = ArrTy->getElementType();
+              llvm::Align Alignment =
+                  CGM.getNaturalTypeAlignment(ETy).getAsAlign();
+              AI->addAttrs(llvm::AttrBuilder().addAlignmentAttr(Alignment));
+              if (!getContext().getTargetAddressSpace(ETy) &&
+                  !CGM.getCodeGenOpts().NullPointerIsValid)
+                AI->addAttr(llvm::Attribute::NonNull);
+            }
           }
 
           // Set `align` attribute if any.
diff --git a/clang/test/CodeGen/vla.c b/clang/test/CodeGen/vla.c
index 16b82f4acc7d..3142050149aa 100644
--- a/clang/test/CodeGen/vla.c
+++ b/clang/test/CodeGen/vla.c
@@ -200,13 +200,13 @@ void test7(int a[b(0)]) {
 // Make sure we emit dereferenceable or nonnull when the static keyword is
 // provided.
 void test8(int a[static 3]) { }
-// CHECK: define void @test8(i32* dereferenceable(12) %a)
+// CHECK: define void @test8(i32* align 4 dereferenceable(12) %a)
 
 void test9(int n, int a[static n]) { }
-// NULL-INVALID: define void @test9(i32 %n, i32* nonnull %a)
-// NULL-VALID: define void @test9(i32 %n, i32* %a)
+// NULL-INVALID: define void @test9(i32 %n, i32* nonnull align 4 %a)
+// NULL-VALID: define void @test9(i32 %n, i32* align 4 %a)
 
 // Make sure a zero-sized static array extent is still required to be nonnull.
 void test10(int a[static 0]) {}
-// NULL-INVALID: define void @test10(i32* nonnull %a)
-// NULL-VALID: define void @test10(i32* %a)
+// NULL-INVALID: define void @test10(i32* nonnull align 4 %a)
+// NULL-VALID: define void @test10(i32* align 4 %a)