[llvm-commits] [llvm] r48356 - /llvm/trunk/lib/Target/X86/README.txt

[Chris Lattner]

On Mar 14, 2008, at 10:15 AM, Evan Cheng wrote:
> This seems like a target independent dag combiner xform?

Turning the first into the second is target independent, and should  
actually be done in instcombine.  The problem is that the second one  
generates worse code from the x86 backend than the first one.

On PPC and ARM, they generate equally good code (on ARM they both  
compile to identical instructions even) so consider this to be "X86  
generates inefficient code for:

define i32 @test2(i32 %f12) {
	%f11 = shl i32 %f12, 8
	%tmp7.25 = ashr i32 %f11, 24
	ret i32 %tmp7.25
}"

:)

-Chris


> On Mar 13, 2008, at 11:00 PM, Chris Lattner wrote:
>
>>
>>
>> +These two functions perform identical operations:
>> +
>> +define i32 @test(i32 %f12) {
>> +	%tmp7.25 = lshr i32 %f12, 16		
>> +	%tmp7.26 = trunc i32 %tmp7.25 to i8
>> +	%tmp78.2 = sext i8 %tmp7.26 to i32
>> +	ret i32 %tmp78.2
>> +}
>> +
>> +define i32 @test2(i32 %f12) {
>> +	%f11 = shl i32 %f12, 8
>> +	%tmp7.25 = ashr i32 %f11, 24
>> +	ret i32 %tmp7.25
>> +}
>> +
>> +but the first compiles into significantly better code on x86-32:
>> +
>> +_test:
>> +	movsbl	6(%esp), %eax
>> +	ret
>> +_test2:
>> +	movl	4(%esp), %eax
>> +	shll	$8, %eax
>> +	sarl	$24, %eax
>> +	ret
>> +
>> +and on x86-64:
>> +
>> +_test:
>> +	shrl	$16, %edi
>> +	movsbl	%dil, %eax
>> +	ret
>> +_test2:
>> +	shll	$8, %edi
>> +	movl	%edi, %eax
>> +	sarl	$24, %eax
>> +	ret
>> +
>> +I would like instcombine to canonicalize the first into the second
>> (since it is
>> +shorter and doesn't involve type width changes) but the x86 backend
>> needs to do
>> +the right thing with the later sequence first.
>> +
>> +//
>> =
>> =
>> =
>> ---------------------------------------------------------------------
>> ===//
>>
>>
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