[cfe-dev] alignment of fields in byval aggregate arguments

[Akira Hatanaka]

I am trying to make changes in the way clang handles Mips' N64 byval
arguments based on the recommendation in the link below. This ABI
requires double arguments in byval structures be passed in 64-bit
double precision registers and everything else in 64-bit integer
registers (or on the stack, if the argument registers are already in
use).

http://thread.gmane.org/gmane.comp.compilers.llvm.devel/44509

This is the example source program I am going to use to explain my approach:

### source code
typedef struct {
  double d1;
  double d2;
  int i1[4];
  double d3;
} S1;

void foo2(int, S1 s1);

void foo1(S1 *s1) {
  foo2(13, *s1);
}
### end of source code

The changes I made are in MipsABIInfo::classifyArgumentType in
lib/CodeGen/TargetInfo.cpp. Instead of returning
ABIArgInfo::getIndirect when a byval argument is passed, it returns
ABIArgInfo::getDirect(structure {f64, f64, i64, i64, f64}).

This is the bit code clang generates after I apply the changes:

### bit code
%struct.S1 = type { double, double, [4 x i32], double }

define void @foo1(%struct.S1* nocapture %s1) nounwind {
entry:
  %0 = getelementptr inbounds %struct.S1* %s1, i64 0, i32 0
  %1 = load double* %0, align 1
  %2 = getelementptr %struct.S1* %s1, i64 0, i32 1
  %3 = load double* %2, align 1
  %4 = getelementptr %struct.S1* %s1, i64 0, i32 2
  %5 = bitcast [4 x i32]* %4 to i64*
  %6 = load i64* %5, align 1
  %7 = getelementptr %struct.S1* %s1, i64 0, i32 2, i64 2
  %8 = bitcast i32* %7 to i64*
  %9 = load i64* %8, align 1
  %10 = getelementptr %struct.S1* %s1, i64 0, i32 3
  %11 = load double* %10, align 1
  tail call void @foo2(i32 13, double %1, double %3, i64 %6, i64 %9,
double %11) nounwind
  ret void
}

declare void @foo2(i32, double, double, i64, i64, double)
### end of bit code

The generated code above is fine except that the loads' alignments are
all 1, which results in the backend generating unaligned loads. I
tracked down what is causing this to happen and discovered the
following piece of code in CGCall.cpp is determining the alignment:

01703           // We don't know what we're loading from.
01704           LI->setAlignment(1);

What is the reason the alignment here always has to be 1 and are there
any ways to have it emit a different alignment?