[ Adding sparse-linux to CC. ]
On Sat, 27 Aug 2011, Linus Torvalds wrote:
On Sat, Aug 27, 2011 at 2:19 AM, Pekka Enberg <penberg@xxxxxxxxxx> wrote:
Looking at this:
sete:
.L0x7f188cf71c90:
<entry-point>
seteq.32 %r83 <- %arg1, %arg2
cast.32 %r84 <- (8) %r83
ret.32 %r84
Why do we have "seteq.32" there but then we have a "cast.32" from %r83 which
is 8 bits? Isn't it linearize.c that's confused here?
No, the code is correct, you misunderstand what "seteq" does.
The 32 in "seteq.32" is the OPERAND WIDTH. It takes two 32-bit values
and checks that they are equal.
The OUTPUT WIDTH is "boolean". Which you changed to 8 bits (to match
x86 semantics). So when you return an "int", you do indeed need to
cast from 8 bits to 32 bits.
There are a few ops that have different operand width from output
width. The cast operation itself is the obvious case, and it shows its
operand/output widths explicitly. But "setcc" is another - since the
output is always just a boolean.
So you were obviously correct. I checked the LLVM bitcode and I was simply
using the wrong type of cast. With this small change:
diff --git a/sparse-llvm.c b/sparse-llvm.c
index f89f7a7..a9bf679 100644
--- a/sparse-llvm.c
+++ b/sparse-llvm.c
@@ -607,7 +607,7 @@ static void output_op_cast(struct function *fn, struct instruction *insn)
if (symbol_is_fp_type(insn->type))
target = LLVMBuildFPCast(fn->builder, src, symbol_type(insn->type), target_name);
else
- target = LLVMBuildIntCast(fn->builder, src, symbol_type(insn->type), target_name);
+ target = LLVMBuildZExt(fn->builder, src, symbol_type(insn->type), target_name);
insn->target->priv = target;
}
the generated code now looks sane:
0000000000000000 <sete>:
0: 39 f7 cmp %esi,%edi
2: 0f 94 c0 sete %al
5: 0f b6 c0 movzbl %al,%eax
8: c3 retq
9: eb 05 jmp 10 <setne>
However, i'm not 100% sure that's sufficient. Is OP_CAST always
zero-extend or do we need to check for something specific here?
Pekka
