AMDGPU/SI: Fold operands with sub-registers

Summary:
Multi-dword constant loads generated unnecessary moves from SGPRs into VGPRs,
increasing the code size and VGPR pressure. These moves are now folded away.

Note that this lack of operand folding was not a problem for VMEM loads,
because COPY nodes from VReg_Nnn to VGPR32 are eliminated by the register
coalescer.

Some tests are updated, note that the fsub.ll test explicitly checks that
the move is elided.

With the IR generated by current Mesa, the changes are obviously relatively
minor:

7063 shaders in 3531 tests
Totals:
SGPRS: 351872 -> 352560 (0.20 %)
VGPRS: 199984 -> 200732 (0.37 %)
Code Size: 9876968 -> 9881112 (0.04 %) bytes
LDS: 91 -> 91 (0.00 %) blocks
Scratch: 1779712 -> 1767424 (-0.69 %) bytes per wave
Wait states: 295164 -> 295337 (0.06 %)

Totals from affected shaders:
SGPRS: 65784 -> 66472 (1.05 %)
VGPRS: 38064 -> 38812 (1.97 %)
Code Size: 1993828 -> 1997972 (0.21 %) bytes
LDS: 42 -> 42 (0.00 %) blocks
Scratch: 795648 -> 783360 (-1.54 %) bytes per wave
Wait states: 54026 -> 54199 (0.32 %)

Reviewers: tstellarAMD, arsenm, mareko

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D15875

llvm-svn: 257074

1 files changed, 1 insertions, 4 deletions

diff --git a/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp b/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
index 02a39307e74..ccbf7c80f2a 100644
--- a/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
+++ b/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
@@ -334,13 +334,10 @@ bool SIFoldOperands::runOnMachineFunction(MachineFunction &MF) {
           !MRI.hasOneUse(MI.getOperand(0).getReg()))
         continue;
 
-      // FIXME: Fold operands with subregs.
       if (OpToFold.isReg() &&
-          (!TargetRegisterInfo::isVirtualRegister(OpToFold.getReg()) ||
-           OpToFold.getSubReg()))
+          !TargetRegisterInfo::isVirtualRegister(OpToFold.getReg()))
         continue;
 
-
       // We need mutate the operands of new mov instructions to add implicit
       // uses of EXEC, but adding them invalidates the use_iterator, so defer
       // this.