[PPC] Use subfic instruction for subtract from immediate

[inouehrs]

Summary

• generate subfic instruction for subtracting a value in a GPR from a 16-bit immediate (e.g. x = 64 - y); currently LLVM generates two instructions (load immediate + subtract) for this purpose
• add a unit test to confirm that subfic is used if possible

[hfinkel]

Please upload the patch with full context (see http://llvm.org/docs/Phabricator.html#requesting-a-review-via-the-web-interface).

[inouehrs]

Uploaded the patch with full context.

[nemanjai]

test/CodeGen/PowerPC/subtract_from_imm.ll

4

Please remove this metadata and add -mtriple to the run line (also add a run line for ppc64le).

[inouehrs]

remove metadata from unit test and add -mtriple to the run lines

[inouehrs]

gentle ping

[nemanjai]

LGTM

lib/Target/PowerPC/PPCInstr64Bit.td

 // amounts.
 def : Pat<(sra i64:$rS, i32:$rB),
           (SRAD $rS, $rB)>;
 def : Pat<(srl i64:$rS, i32:$rB),
           (SRD $rS, $rB)>;
 def : Pat<(shl i64:$rS, i32:$rB),
           (SLD $rS, $rB)>;
 
+// SUBFIC
+def : Pat<(sub imm64SExt16:$imm, i64:$in),
+          (SUBFIC8 $in, imm:$imm)>;
+
 // SHL/SRL
 def : Pat<(shl i64:$in, (i32 imm:$imm)),
           (RLDICR $in, imm:$imm, (SHL64 imm:$imm))>;
 def : Pat<(srl i64:$in, (i32 imm:$imm)),
           (RLDICL $in, (SRL64 imm:$imm), imm:$imm)>;
 
 // ROTL
 def : Pat<(rotl i64:$in, i32:$sh),

test/CodeGen/PowerPC/subtract_from_imm.ll

+; RUN: llc -verify-machineinstrs -mtriple=powerpc64le-unknown-linux-gnu < %s | FileCheck %s
+; RUN: llc -verify-machineinstrs -mtriple=powerpc64-unknown-linux-gnu < %s | FileCheck %s
+
+; Make sure that the subfic is generated iff possible
+
+define i64 @subtract_from_imm1(i64 %v) nounwind readnone {
+entry:
+; CHECK-LABEL: subtract_from_imm1
+; CHECK: subfic 3, 3, 32767
+; CHECK: blr
+  %sub = sub i64 32767, %v
+  ret i64 %sub
+}
+
+define i64 @subtract_from_imm2(i64 %v) nounwind readnone {
+entry:
+; CHECK-LABEL: subtract_from_imm2
+; CHECK-NOT: subfic
+; CHECK: blr
+  %sub = sub i64 32768, %v
+  ret i64 %sub
+}
+
+define i64 @subtract_from_imm3(i64 %v) nounwind readnone {
+entry:
+; CHECK-LABEL: subtract_from_imm3
+; CHECK: subfic 3, 3, -32768
+; CHECK: blr
+  %sub = sub i64 -32768, %v
+  ret i64 %sub
+}
+
+define i64 @subtract_from_imm4(i64 %v) nounwind readnone {
+entry:
+; CHECK-LABEL: subtract_from_imm4
+; CHECK-NOT: subfic
+; CHECK: blr
+  %sub = sub i64 -32769, %v
+  ret i64 %sub
+}
+