[llvm] [ShrinkWrap] Consider constant pool access as non-stack access (PR #164393)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-x86

Author: Sushant Gokhale (sushgokh)

<details>
<summary>Changes</summary>

As far as I understand, constant pool access does not access stack and accesses read-only memory.

This patch considers constant pool access as non-stack access allowing shrink wrapping to happen in the concerned test.

We should be seeing perf improvement with povray benchmark from SPEC17(around 12% with -flto -Ofast) after this patch.


An NFC PR #<!-- -->162476 already exists to upload the test before the patch but approval has got delayed. So, as @<!-- -->davemgreen suggested in that PR, I have uploaded the test and patch in this single PR to show how test looks like.

---
Full diff: https://github.com/llvm/llvm-project/pull/164393.diff


3 Files Affected:

- (modified) llvm/lib/CodeGen/ShrinkWrap.cpp (+1-1) 
- (added) llvm/test/CodeGen/AArch64/shrink-wrap-const-pool-access.mir (+75) 
- (modified) llvm/test/CodeGen/X86/fp128-select.ll (+3-3) 


``````````diff
diff --git a/llvm/lib/CodeGen/ShrinkWrap.cpp b/llvm/lib/CodeGen/ShrinkWrap.cpp
index 826e4126de44c..83581052560cb 100644
--- a/llvm/lib/CodeGen/ShrinkWrap.cpp
+++ b/llvm/lib/CodeGen/ShrinkWrap.cpp
@@ -319,7 +319,7 @@ bool ShrinkWrapImpl::useOrDefCSROrFI(const MachineInstr &MI, RegScavenger *RS,
       return isa<GlobalValue>(UO);
     }
     if (const PseudoSourceValue *PSV = Op->getPseudoValue())
-      return PSV->isJumpTable();
+      return PSV->isJumpTable() || PSV->isConstantPool();
     return false;
   };
   // Load/store operations may access the stack indirectly when we previously
diff --git a/llvm/test/CodeGen/AArch64/shrink-wrap-const-pool-access.mir b/llvm/test/CodeGen/AArch64/shrink-wrap-const-pool-access.mir
new file mode 100644
index 0000000000000..581e8b3f299ee
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/shrink-wrap-const-pool-access.mir
@@ -0,0 +1,75 @@
+# RUN: llc -simplify-mir -run-pass=shrink-wrap -o - %s | FileCheck %s
+--- |
+  declare double @foo()
+
+  define double @shrink_wrap_load_from_const_pool(double %q) {
+  entry:
+    %0 = fcmp oeq double %q, 3.125500e+02
+    br i1 %0, label %common.ret, label %if.else
+
+  common.ret:                                       ; preds = %if.else, %entry, %exit1
+    %common.ret.op = phi double [ %3, %exit1 ], [ 0.000000e+00, %entry ], [ 0.000000e+00, %if.else ]
+    ret double %common.ret.op
+
+  if.else:                                          ; preds = %entry
+    %1 = call double @foo()
+    %2 = fcmp oeq double %1, 0.000000e+00
+    br i1 %2, label %exit1, label %common.ret
+
+  exit1:                                            ; preds = %if.else
+    %3 = call double @foo()
+    br label %common.ret
+  }
+...
+# Following code has a load from constant pool. Accessing constant pool
+# must not be considered as a stack access and hence, shrink wrapping must
+# happen.
+# CHECK:  savePoint:
+# CHECK:    - point:           '%bb.3'
+# CHECK:  restorePoint:
+# CHECK:    - point:           '%bb.5'
+---
+name:            shrink_wrap_load_from_const_pool
+tracksRegLiveness: true
+constants:
+  - id:              0
+    value:           'double 3.125500e+02'
+    alignment:       8
+body:             |
+  bb.0.entry:
+    successors: %bb.4(0x50000000), %bb.2(0x30000000)
+    liveins: $d0
+
+    renamable $d1 = COPY $d0
+    renamable $x8 = ADRP target-flags(aarch64-page) %const.0
+    renamable $d2 = LDRDui killed renamable $x8, target-flags(aarch64-pageoff, aarch64-nc) %const.0 :: (load (s64) from constant-pool)
+    renamable $d0 = FMOVD0
+    nofpexcept FCMPDrr killed renamable $d1, killed renamable $d2, implicit-def $nzcv, implicit $fpcr
+    Bcc 1, %bb.2, implicit killed $nzcv
+
+  bb.4:
+    liveins: $d0
+
+  bb.1.common.ret:
+    liveins: $d0
+
+    RET_ReallyLR implicit $d0
+
+  bb.2.if.else:
+    successors: %bb.3(0x50000000), %bb.1(0x30000000)
+
+    ADJCALLSTACKDOWN 0, 0, implicit-def dead $sp, implicit $sp
+    BL @foo, csr_aarch64_aapcs, implicit-def dead $lr, implicit $sp, implicit-def $sp, implicit-def $d0
+    ADJCALLSTACKUP 0, 0, implicit-def dead $sp, implicit $sp
+    renamable $d1 = COPY $d0
+    renamable $d0 = FMOVD0
+    nofpexcept FCMPDri killed renamable $d1, implicit-def $nzcv, implicit $fpcr
+    Bcc 1, %bb.1, implicit killed $nzcv
+    B %bb.3
+
+  bb.3.exit1:
+    ADJCALLSTACKDOWN 0, 0, implicit-def dead $sp, implicit $sp
+    BL @foo, csr_aarch64_aapcs, implicit-def dead $lr, implicit $sp, implicit-def $sp, implicit-def $d0
+    ADJCALLSTACKUP 0, 0, implicit-def dead $sp, implicit $sp
+    B %bb.1
+...
diff --git a/llvm/test/CodeGen/X86/fp128-select.ll b/llvm/test/CodeGen/X86/fp128-select.ll
index 659e4ddedc646..27a651e23f886 100644
--- a/llvm/test/CodeGen/X86/fp128-select.ll
+++ b/llvm/test/CodeGen/X86/fp128-select.ll
@@ -13,8 +13,8 @@ define void @test_select(ptr %p, ptr %q, i1 zeroext %c) nounwind {
 ; SSE:       # %bb.0:
 ; SSE-NEXT:    testl %edx, %edx
 ; SSE-NEXT:    jne .LBB0_1
-; SSE-NEXT:  # %bb.3:
-; SSE-NEXT:    movaps {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm0
+; SSE-NEXT:  # %bb.2:
+; SSE-NEXT:    movaps {{.*#+}} xmm0 = [NaN]
 ; SSE-NEXT:    movaps %xmm0, (%rsi)
 ; SSE-NEXT:    retq
 ; SSE-NEXT:  .LBB0_1:
@@ -58,7 +58,7 @@ define fp128 @test_select_cc(fp128, fp128) nounwind {
 ; SSE-NEXT:    xorps %xmm1, %xmm1
 ; SSE-NEXT:    jmp .LBB1_3
 ; SSE-NEXT:  .LBB1_1:
-; SSE-NEXT:    movaps {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm1
+; SSE-NEXT:    movaps {{.*#+}} xmm1 = [1.0E+0]
 ; SSE-NEXT:  .LBB1_3: # %BB0
 ; SSE-NEXT:    testl %ebx, %ebx
 ; SSE-NEXT:    movaps (%rsp), %xmm0 # 16-byte Reload

``````````

</details>


https://github.com/llvm/llvm-project/pull/164393