[ARM] Fix over-alignment in arguments that are HA of 128-bit vectors

Code in `CC_ARM_AAPCS_Custom_Aggregate()` is responsible for handling
homogeneous aggregates for `CC_ARM_AAPCS_VFP`. When an aggregate ends up
fully on stack, the function tries to pack all resulting items of the
aggregate as tightly as possible according to AAPCS.

Once the first item was laid out, the alignment used for consecutive
items was the size of one item. This logic went wrong for 128-bit
vectors because their alignment is normally only 64 bits, and so could
result in inserting unexpected padding between the first and second
element.

The patch fixes the problem by updating the alignment with the item size
only if this results in reducing it.

Differential Revision: https://reviews.llvm.org/D49720

llvm-svn: 338233

2 files changed, 22 insertions, 5 deletions

diff --git a/llvm/lib/Target/ARM/ARMCallingConv.h b/llvm/lib/Target/ARM/ARMCallingConv.h
index 63bf48abb7a..543165de38d 100644
--- a/llvm/lib/Target/ARM/ARMCallingConv.h
+++ b/llvm/lib/Target/ARM/ARMCallingConv.h
@@ -269,14 +269,15 @@ static bool CC_ARM_AAPCS_Custom_Aggregate(unsigned &ValNo, MVT &ValVT,
   for (auto Reg : RegList)
     State.AllocateReg(Reg);
 
+  // After the first item has been allocated, the rest are packed as tightly as
+  // possible. (E.g. an incoming i64 would have starting Align of 8, but we'll
+  // be allocating a bunch of i32 slots).
+  unsigned RestAlign = std::min(Align, Size);
+
   for (auto &It : PendingMembers) {
     It.convertToMem(State.AllocateStack(Size, Align));
     State.addLoc(It);
-
-    // After the first item has been allocated, the rest are packed as tightly
-    // as possible. (E.g. an incoming i64 would have starting Align of 8, but
-    // we'll be allocating a bunch of i32 slots).
-    Align = Size;
+    Align = RestAlign;
   }
 
   // All pending members have now been allocated
diff --git a/llvm/test/CodeGen/ARM/aggregate-padding.ll b/llvm/test/CodeGen/ARM/aggregate-padding.ll
index bc46a9cdf91..ae7ab90fcd2 100644
--- a/llvm/test/CodeGen/ARM/aggregate-padding.ll
+++ b/llvm/test/CodeGen/ARM/aggregate-padding.ll
@@ -99,3 +99,19 @@ define i16 @test_i16_forced_stack([8 x double], double, i32, i32, [3 x i16] %arg
   %sum = add i16 %val0, %val2
   ret i16 %sum
 }
+
+; [2 x <4 x i32>] should be aligned only on a 64-bit boundary and contiguous.
+; None of the two <4 x i32> elements should introduce any padding to 128 bits.
+define i32 @test_4xi32_64bit_aligned_and_contiguous([8 x double], float, [2 x <4 x i32>] %arg) nounwind {
+; CHECK-LABEL: test_4xi32_64bit_aligned_and_contiguous:
+; CHECK-DAG: ldr [[VAL0_0:r[0-9]+]], [sp, #8]
+; CHECK-DAG: ldr [[VAL1_0:r[0-9]+]], [sp, #24]
+; CHECK: add r0, [[VAL0_0]], [[VAL1_0]]
+
+  %val0 = extractvalue [2 x <4 x i32>] %arg, 0
+  %val0_0 = extractelement <4 x i32> %val0, i32 0
+  %val1 = extractvalue [2 x <4 x i32>] %arg, 1
+  %val1_0 = extractelement <4 x i32> %val1, i32 0
+  %sum = add i32 %val0_0, %val1_0
+  ret i32 %sum
+}