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We want to do this for 2 reasons: 1. Value tracking does not recognize the ashr variant, so it would fail to match for cases like D39766. 2. DAGCombiner does better at producing optimal codegen when we have the cmp+sel pattern. More detail about what happens in the backend: 1. DAGCombiner has a generic transform for all targets to convert the scalar cmp+sel variant of abs into the shift variant. That is the opposite of this IR canonicalization. 2. DAGCombiner has a generic transform for all targets to convert the vector cmp+sel variant of abs into either an ABS node or the shift variant. That is again the opposite of this IR canonicalization. 3. DAGCombiner has a generic transform for all targets to convert the exact shift variants produced by #1 or #2 into an ISD::ABS node. Note: It would be an efficiency improvement if we had #1 go directly to an ABS node when that's legal/custom. 4. The pattern matching above is incomplete, so it is possible to escape the intended/optimal codegen in a variety of ways. a. For #2, the vector path is missing the case for setlt with a '1' constant. b. For #3, we are missing a match for commuted versions of the shift variants. 5. Therefore, this IR canonicalization can only help get us to the optimal codegen. The version of cmp+sel produced by this patch will be recognized in the DAG and converted to an ABS node when possible or the shift sequence when not. 6. In the following examples with this patch applied, we may get conditional moves rather than the shift produced by the generic DAGCombiner transforms. The conditional move is created using a target-specific decision for any given target. Whether it is optimal or not for a particular subtarget may be up for debate. define i32 @abs_shifty(i32 %x) { %signbit = ashr i32 %x, 31 %add = add i32 %signbit, %x %abs = xor i32 %signbit, %add ret i32 %abs } define i32 @abs_cmpsubsel(i32 %x) { %cmp = icmp slt i32 %x, zeroinitializer %sub = sub i32 zeroinitializer, %x %abs = select i1 %cmp, i32 %sub, i32 %x ret i32 %abs } define <4 x i32> @abs_shifty_vec(<4 x i32> %x) { %signbit = ashr <4 x i32> %x, <i32 31, i32 31, i32 31, i32 31> %add = add <4 x i32> %signbit, %x %abs = xor <4 x i32> %signbit, %add ret <4 x i32> %abs } define <4 x i32> @abs_cmpsubsel_vec(<4 x i32> %x) { %cmp = icmp slt <4 x i32> %x, zeroinitializer %sub = sub <4 x i32> zeroinitializer, %x %abs = select <4 x i1> %cmp, <4 x i32> %sub, <4 x i32> %x ret <4 x i32> %abs } > $ ./opt -instcombine shiftyabs.ll -S | ./llc -o - -mtriple=x86_64 -mattr=avx > abs_shifty: > movl %edi, %eax > negl %eax > cmovll %edi, %eax > retq > > abs_cmpsubsel: > movl %edi, %eax > negl %eax > cmovll %edi, %eax > retq > > abs_shifty_vec: > vpabsd %xmm0, %xmm0 > retq > > abs_cmpsubsel_vec: > vpabsd %xmm0, %xmm0 > retq > > $ ./opt -instcombine shiftyabs.ll -S | ./llc -o - -mtriple=aarch64 > abs_shifty: > cmp w0, #0 // =0 > cneg w0, w0, mi > ret > > abs_cmpsubsel: > cmp w0, #0 // =0 > cneg w0, w0, mi > ret > > abs_shifty_vec: > abs v0.4s, v0.4s > ret > > abs_cmpsubsel_vec: > abs v0.4s, v0.4s > ret > > $ ./opt -instcombine shiftyabs.ll -S | ./llc -o - -mtriple=powerpc64le > abs_shifty: > srawi 4, 3, 31 > add 3, 3, 4 > xor 3, 3, 4 > blr > > abs_cmpsubsel: > srawi 4, 3, 31 > add 3, 3, 4 > xor 3, 3, 4 > blr > > abs_shifty_vec: > vspltisw 3, -16 > vspltisw 4, 15 > vsubuwm 3, 4, 3 > vsraw 3, 2, 3 > vadduwm 2, 2, 3 > xxlxor 34, 34, 35 > blr > > abs_cmpsubsel_vec: > vspltisw 3, -16 > vspltisw 4, 15 > vsubuwm 3, 4, 3 > vsraw 3, 2, 3 > vadduwm 2, 2, 3 > xxlxor 34, 34, 35 > blr > Differential Revision: https://reviews.llvm.org/D40984 llvm-svn: 320921 |
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