Commit Graph

5 Commits

Author SHA1 Message Date
Chris Lattner
d43d00cf3a Significantly simplify and improve handling of FP function results on x86-32.
This case returns the value in ST(0) and then has to convert it to an SSE
register.  This causes significant codegen ugliness in some cases.  For 
example in the trivial fp-stack-direct-ret.ll testcase we used to generate:

_bar:
	subl	$28, %esp
	call	L_foo$stub
	fstpl	16(%esp)
	movsd	16(%esp), %xmm0
	movsd	%xmm0, 8(%esp)
	fldl	8(%esp)
	addl	$28, %esp
	ret

because we move the result of foo() into an XMM register, then have to
move it back for the return of bar.

Instead of hacking ever-more special cases into the call result lowering code
we take a much simpler approach: on x86-32, fp return is modeled as always 
returning into an f80 register which is then truncated to f32 or f64 as needed.
Similarly for a result, we model it as an extension to f80 + return.

This exposes the truncate and extensions to the dag combiner, allowing target
independent code to hack on them, eliminating them in this case.  This gives 
us this code for the example above:

_bar:
	subl	$12, %esp
	call	L_foo$stub
	addl	$12, %esp
	ret

The nasty aspect of this is that these conversions are not legal, but we want
the second pass of dag combiner (post-legalize) to be able to hack on them.
To handle this, we lie to legalize and say they are legal, then custom expand
them on entry to the isel pass (PreprocessForFPConvert).  This is gross, but
less gross than the code it is replacing :)

This also allows us to generate better code in several other cases.  For 
example on fp-stack-ret-conv.ll, we now generate:

_test:
	subl	$12, %esp
	call	L_foo$stub
	fstps	8(%esp)
	movl	16(%esp), %eax
	cvtss2sd	8(%esp), %xmm0
	movsd	%xmm0, (%eax)
	addl	$12, %esp
	ret

where before we produced (incidentally, the old bad code is identical to what
gcc produces):

_test:
	subl	$12, %esp
	call	L_foo$stub
	fstpl	(%esp)
	cvtsd2ss	(%esp), %xmm0
	cvtss2sd	%xmm0, %xmm0
	movl	16(%esp), %eax
	movsd	%xmm0, (%eax)
	addl	$12, %esp
	ret

Note that we generate slightly worse code on pr1505b.ll due to a scheduling 
deficiency that is unrelated to this patch.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46307 91177308-0d34-0410-b5e6-96231b3b80d8
2008-01-24 08:07:48 +00:00
Chris Lattner
7323999b31 take these with a pr #
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46303 91177308-0d34-0410-b5e6-96231b3b80d8
2008-01-24 06:35:44 +00:00
Evan Cheng
9c9cec4372 Codegen improvement has reduced one spill.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45814 91177308-0d34-0410-b5e6-96231b3b80d8
2008-01-10 02:54:40 +00:00
Dan Gohman
28beeeac4d Convert tests using "| wc -l | grep ..." to use the count script.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@41097 91177308-0d34-0410-b5e6-96231b3b80d8
2007-08-15 13:36:28 +00:00
Dale Johannesen
9eed80cb12 New testcases for rev 37847 (PR's 1489 and 1505).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37848 91177308-0d34-0410-b5e6-96231b3b80d8
2007-07-03 00:58:37 +00:00