Add a note
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28401 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index ef819d7..34055dd 100644
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -539,3 +539,41 @@
ret
//===---------------------------------------------------------------------===//
+
+Consider this:
+
+typedef struct pair { float A, B; } pair;
+void pairtest(pair P, float *FP) {
+ *FP = P.A+P.B;
+}
+
+We currently generate this code with llvmgcc4:
+
+_pairtest:
+ subl $12, %esp
+ movl 20(%esp), %eax
+ movl %eax, 4(%esp)
+ movl 16(%esp), %eax
+ movl %eax, (%esp)
+ movss (%esp), %xmm0
+ addss 4(%esp), %xmm0
+ movl 24(%esp), %eax
+ movss %xmm0, (%eax)
+ addl $12, %esp
+ ret
+
+we should be able to generate:
+_pairtest:
+ movss 4(%esp), %xmm0
+ movl 12(%esp), %eax
+ addss 8(%esp), %xmm0
+ movss %xmm0, (%eax)
+ ret
+
+The issue is that llvmgcc4 is forcing the struct to memory, then passing it as
+integer chunks. It does this so that structs like {short,short} are passed in
+a single 32-bit integer stack slot. We should handle the safe cases above much
+nicer, while still handling the hard cases.
+
+//===---------------------------------------------------------------------===//
+