[llvm] [X86InstrInfo] support memfold on spillable inline asm (PR #70832)

Nick Desaulniers via llvm-commits llvm-commits at lists.llvm.org
Tue Nov 21 14:46:21 PST 2023


================
@@ -0,0 +1,232 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 3
+# RUN: llc -start-after=finalize-isel -regalloc=greedy -stop-after=greedy \
+# RUN:   -verify-machineinstrs -verify-regalloc %s -o - | FileCheck %s
+--- |
+  target datalayout = "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-i128:128-f64:32:64-f80:32-n8:16:32-S128"
+  target triple = "i386-unknown-linux-gnu"
+
+  define void @input(i32 %0) #0 {
+    call void asm "# $0", "rm,~{ax},~{cx},~{dx},~{si},~{di},~{bx},~{bp}"(i32 %0)
+    ret void
+  }
+
+  define i32 @output() #0 {
+    %1 = alloca i32, align 4
+    call void asm "# $0", "=*rm,~{ax},~{cx},~{dx},~{si},~{di},~{bx},~{bp}"(ptr nonnull elementtype(i32) %1)
+    %2 = load i32, ptr %1, align 4
+    ret i32 %2
+  }
+
+  define i32 @inout(i32 %0) #0 {
+    %2 = alloca i32, align 4
+    store i32 %0, ptr %2, align 4
+    call void asm "# $0 $1", "=*rm,0,~{ax},~{cx},~{dx},~{si},~{di},~{bx},~{bp}"(ptr nonnull elementtype(i32) %2, i32 %0)
+    %3 = load i32, ptr %2, align 4
+    ret i32 %3
+  }
+
+  attributes #0 = { nounwind }
+
+...
+---
+name:            input
+alignment:       16
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+callsEHReturn:   false
+callsUnwindInit: false
+hasEHCatchret:   false
+hasEHScopes:     false
+hasEHFunclets:   false
+isOutlined:      false
+debugInstrRef:   false
+failsVerification: false
+tracksDebugUserValues: false
+registers:
+  - { id: 0, class: gr32, preferred-register: '' }
+liveins:         []
+frameInfo:
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    4
+  adjustsStack:    false
+  hasCalls:        false
+  stackProtector:  ''
+  functionContext: ''
+  maxCallFrameSize: 4294967295
+  cvBytesOfCalleeSavedRegisters: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+  hasTailCall:     false
+  localFrameSize:  0
+  savePoint:       ''
+  restorePoint:    ''
+fixedStack:
+  - { id: 0, type: default, offset: 0, size: 4, alignment: 16, stack-id: default,
+      isImmutable: true, isAliased: false, callee-saved-register: '', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+stack:           []
+entry_values:    []
+callSites:       []
+debugValueSubstitutions: []
+constants:       []
+machineFunctionInfo: {}
+body:             |
+  bb.0 (%ir-block.1):
+    ; CHECK-LABEL: name: input
+    ; CHECK: INLINEASM &"# $0", 8 /* mayload attdialect */, 262190 /* mem:m */, %fixed-stack.0, 1, $noreg, 0, $noreg, 12 /* clobber */, implicit-def dead early-clobber $ax, 12 /* clobber */, implicit-def dead early-clobber $cx, 12 /* clobber */, implicit-def dead early-clobber $dx, 12 /* clobber */, implicit-def dead early-clobber $si, 12 /* clobber */, implicit-def dead early-clobber $di, 12 /* clobber */, implicit-def dead early-clobber $bx, 12 /* clobber */, implicit-def dead early-clobber $bp :: (load (s32) from %fixed-stack.0, align 16)
+    ; CHECK-NEXT: RET 0
+    %0:gr32 = MOV32rm %fixed-stack.0, 1, $noreg, 0, $noreg :: (load (s32) from %fixed-stack.0, align 16)
+    INLINEASM &"# $0", 0 /* attdialect */, 1076101129 /* reguse:GR32 spillable */, %0, 12 /* clobber */, implicit-def early-clobber $ax, 12 /* clobber */, implicit-def early-clobber $cx, 12 /* clobber */, implicit-def early-clobber $dx, 12 /* clobber */, implicit-def early-clobber $si, 12 /* clobber */, implicit-def early-clobber $di, 12 /* clobber */, implicit-def early-clobber $bx, 12 /* clobber */, implicit-def early-clobber $bp
+    RET 0
+
+...
+---
+name:            output
+alignment:       16
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+callsEHReturn:   false
+callsUnwindInit: false
+hasEHCatchret:   false
+hasEHScopes:     false
+hasEHFunclets:   false
+isOutlined:      false
+debugInstrRef:   false
+failsVerification: false
+tracksDebugUserValues: false
+registers:
+  - { id: 0, class: gr32, preferred-register: '' }
+liveins:         []
+frameInfo:
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    4
+  adjustsStack:    false
+  hasCalls:        false
+  stackProtector:  ''
+  functionContext: ''
+  maxCallFrameSize: 4294967295
+  cvBytesOfCalleeSavedRegisters: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+  hasTailCall:     false
+  localFrameSize:  0
+  savePoint:       ''
+  restorePoint:    ''
+fixedStack:      []
+stack:
+  - { id: 0, name: '', type: default, offset: 0, size: 4, alignment: 4,
+      stack-id: default, callee-saved-register: '', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+entry_values:    []
+callSites:       []
+debugValueSubstitutions: []
+constants:       []
+machineFunctionInfo: {}
+body:             |
+  bb.0 (%ir-block.0):
+    ; CHECK-LABEL: name: output
+    ; CHECK: INLINEASM &"# $0", 16 /* maystore attdialect */, 262190 /* mem:m */, %stack.1, 1, $noreg, 0, $noreg, 12 /* clobber */, implicit-def dead early-clobber $ax, 12 /* clobber */, implicit-def dead early-clobber $cx, 12 /* clobber */, implicit-def dead early-clobber $dx, 12 /* clobber */, implicit-def dead early-clobber $si, 12 /* clobber */, implicit-def dead early-clobber $di, 12 /* clobber */, implicit-def dead early-clobber $bx, 12 /* clobber */, implicit-def dead early-clobber $bp :: (store (s32) into %stack.1)
+    ; CHECK-NEXT: [[MOV32rm:%[0-9]+]]:gr32 = MOV32rm %stack.1, 1, $noreg, 0, $noreg :: (load (s32) from %stack.1)
+    ; CHECK-NEXT: MOV32mr %stack.0, 1, $noreg, 0, $noreg, [[MOV32rm]] :: (store (s32) into %ir.1)
----------------
nickdesaulniers wrote:

Yes, the load and store is redundant.  The INLINEASM could have just stored to `%stack.0`, eliding the store to `%stack.1`, load from `%stack.1`, then store to `%stack.0`.  The code gen when there is memory pressure is sub-optimal because of the redundant store/load.

But it is greedy that determines that a spill is necessary, and creates the slot to spill to (in this case `%stack.1`).  The code I add is just told "use `%stack.1`).

Could we optimize this case later, yes, but I don't think that should be part of this commit.

I guess I'm curious why there isn't a later pass that determines that:
```
(store (s32) into %stack.1)
(load (s32) from %stack.1)
(store (s32) into %ir.1)
```
can't be collapsed into:
```
(store (s32) into %ir.1)
```
Is there a pass that does so, but maybe bails when one of the instructions is INLINEASM?  Or is it too late to know that nothing else depends on that initial store?

https://github.com/llvm/llvm-project/pull/70832


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