AMDGPU/SI: Don't mark VINTRP instructions as mayLoad

[• tstellarAMD]

Summary

These instructions technically do read from memory, but the memory
is considered to be out of bounds for normal load/store instructions.

shader-db stats:

SGPRS: 1416075 -> 1413323 (-0.19 %)
VGPRS: 867413 -> 863935 (-0.40 %)
Spilled SGPRs: 1409 -> 1354 (-3.90 %)
Spilled VGPRs: 63 -> 63 (0.00 %)
Private memory VGPRs: 880 -> 880 (0.00 %)
Scratch size: 2648 -> 2632 (-0.60 %) dwords per thread
Code Size: 37889052 -> 37897340 (0.02 %) bytes
LDS: 2147 -> 2147 (0.00 %) blocks
Max Waves: 279243 -> 280369 (0.40 %)
Wait states: 0 -> 0 (0.00 %)

[arsenm]

what about adding a readonly mem operand?

[mareko]

The portion of LDS dedicated to VINTRP instructions is read-only immutable memory. LDS instruction can't access it, because the VINTRP portion of LDS memory is stolen from the shader. I don't see a reason to mark VINTRP as read-only memory instructions.

[• tstellarAMD]

In D27593#617664, @arsenm wrote:

what about adding a readonly mem operand?

I considered doing that, but I couldn't think of any advantage of doing it that way.

[arsenm]

LGTM

lib/Target/AMDGPU/SIInstrFormats.td

class EXPe : Enc64 {
   let Inst{55-48} = vsrc2;
   let Inst{63-56} = vsrc3;
 }
 
 let Uses = [EXEC] in {
 
 class VINTRPCommon <dag outs, dag ins, string asm, list<dag> pattern> :
     InstSI <outs, ins, asm, pattern> {
-  let mayLoad = 1;
+
+  // VINTRP instructions read parameter values from LDS, but these parameter
+  // values are stored outside of the LDS memory that is allocated to the
+  // shader for general purpose use.
+  //
+  // While it may be possible for ds_read/ds_write instructions to access
+  // the parameter values in LDS, this would essentially be an out-of-bounds
+  // memory access which we consider to be undefined behavior.
+  //
+  // So even though these instructions read memory, this memory is outside the
+  // addressable memory space for the shader, and we consider these instructions
+  // to be readnone.
+  let mayLoad = 0;
   let mayStore = 0;
   let hasSideEffects = 0;
 }
 
 class EXPCommon<dag outs, dag ins, string asm, list<dag> pattern> :
   InstSI<outs, ins, asm, pattern> {
   let EXP = 1;
   let EXP_CNT = 1;

test/CodeGen/AMDGPU/llvm.amdgcn.interp.ll

 ;RUN: llc < %s -march=amdgcn -mcpu=verde -verify-machineinstrs | FileCheck --check-prefix=GCN %s
-;RUN: llc < %s -march=amdgcn -mcpu=tonga -verify-machineinstrs | FileCheck --check-prefix=GCN %s
+;RUN: llc < %s -march=amdgcn -mcpu=tonga -verify-machineinstrs | FileCheck --check-prefixes=GCN,VI %s
 
 ;GCN-LABEL: {{^}}v_interp:
 ;GCN-NOT: s_wqm
 ;GCN: s_mov_b32 m0, s{{[0-9]+}}
 ;GCN: v_interp_p1_f32
 ;GCN: v_interp_p2_f32
 define amdgpu_ps void @v_interp(<16 x i8> addrspace(2)* inreg, <16 x i8> addrspace(2)* inreg, <32 x i8> addrspace(2)* inreg, i32 inreg, <2 x float>) {
 main_body:
   %i = extractelement <2 x float> %4, i32 0
   %j = extractelement <2 x float> %4, i32 1
   %p0_0 = call float @llvm.amdgcn.interp.p1(float %i, i32 0, i32 0, i32 %3)
   %p1_0 = call float @llvm.amdgcn.interp.p2(float %p0_0, float %j, i32 0, i32 0, i32 %3)
   %p0_1 = call float @llvm.amdgcn.interp.p1(float %i, i32 1, i32 0, i32 %3)
   %p1_1 = call float @llvm.amdgcn.interp.p2(float %p0_1, float %j, i32 1, i32 0, i32 %3)
   %const = call float @llvm.amdgcn.interp.mov(i32 2, i32 0, i32 0, i32 %3)
   %w = fadd float %p1_1, %const
   call void @llvm.SI.export(i32 15, i32 1, i32 1, i32 0, i32 1, float %p0_0, float %p0_0, float %p1_1, float %w)
   ret void
 }
 
+; SI won't merge ds memory operations, because of the signed offset bug, so
+; we only have check lines for VI.
+; VI-LABEL: v_interp_readnone:
+; VI: v_mov_b32_e32 [[ZERO:v[0-9]+]], 0
+; VI: ds_write2_b32 v{{[0-9]+}}, [[ZERO]], [[ZERO]] offset1:4
+define amdgpu_ps void @v_interp_readnone(float addrspace(3)* %lds) {
+  store float 0.0, float addrspace(3)* %lds
+  %tmp1 = call float @llvm.amdgcn.interp.mov(i32 2, i32 0, i32 0, i32 0)
+  %tmp2 = getelementptr float, float addrspace(3)* %lds, i32 4
+  store float 0.0, float addrspace(3)* %tmp2
+  call void @llvm.SI.export(i32 15, i32 1, i32 1, i32 0, i32 1, float %tmp1, float %tmp1, float %tmp1, float %tmp1)
+  ret void
+}
+
 ; Function Attrs: nounwind readnone
 declare float @llvm.amdgcn.interp.p1(float, i32, i32, i32) #0
 
 ; Function Attrs: nounwind readnone
 declare float @llvm.amdgcn.interp.p2(float, float, i32, i32, i32) #0
 
 declare float @llvm.amdgcn.interp.mov(i32, i32, i32, i32) #0
 
 declare void @llvm.SI.export(i32, i32, i32, i32, i32, float, float, float, float)
 
 attributes #0 = { nounwind readnone }