[llvm-commits] [llvm] r130206 - in /llvm/trunk: lib/Transforms/Scalar/GVN.cpp test/Transforms/GVN/rle.ll test/Transforms/SimplifyCFG/2006-08-03-Crash.ll

[Chris Lattner]

On Apr 26, 2011, at 12:14 PM, Roman Divacky wrote:
> this is causing some strange regressions here (with greedy regalloc but
> otherwise vanilla llvm):
> 
> before:
> 
> cmpb    $69, 41(%esp)
> 
> after:
> 
> movl    %eax, %ecx
> movb    %ch, %cl
> cmpb    $69, %cl
> 
> and some more

That is pretty nasty codegen, but GVN is doing the right thing here, it is trading a load for some trivial integer operations.  This may not be the best for code size, but it is the right thing to do.  FWIW, this is one in a long series of patches I'm working on to fix PR6627 for you.

Here is what the code looked like before (after straightening out control flow):

	movl	40(%esp), %eax
	cmpw	$267, %ax               # imm = 0x10B
	jne	.LBB7_9
...
.LBB7_9:                                # %if.else
	cmpb	$127, %al
	jne	.LBB7_4
# BB#10:                                # %land.lhs.true
	cmpb	$69, 41(%esp)
	jne	.LBB7_4
# BB#11:                                # %land.lhs.true49
	cmpb	$76, 42(%esp)
	jne	.LBB7_4
# BB#12:                                # %land.lhs.true57
	cmpb	$70, 43(%esp)
	jne	.LBB7_4
# BB#13:                                # %if.then65

Now we have:

	movl	40(%esp), %eax
	cmpw	$267, %ax               # imm = 0x10B
	jne	.LBB7_9
...
.LBB7_9:                                # %if.else
	movl	%eax, %ecx
	cmpb	$127, %cl
	jne	.LBB7_4
# BB#10:                                # %if.else
	movl	%eax, %ecx
	movb	%ch, %cl
	cmpb	$69, %cl
	jne	.LBB7_4
# BB#11:                                # %if.else
	movl	%eax, %ecx
	shrl	$16, %ecx
	cmpb	$76, %cl
	jne	.LBB7_4
# BB#12:                                # %land.lhs.true57
	andl	$-16777216, %eax        # imm = 0xFFFFFFFFFF000000
	cmpl	$1174405120, %eax       # imm = 0x46000000
	jne	.LBB7_4


This instcombine patch is the next step along the way (which will probably improve this, but not get us all the way to the goal), but is causing some codegen pessimizations that I have to deal with before it can land:

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-Chris