[WebAssembly] improve getRegForPromotedValue to avoid meanless value copy (#80469)

When promoted value, it is meaningless to copy value from reg to another
reg with the same type.
This PR add additional check for this cases to reduce the code size.
Fixes: #80053.

Author: HerrCai0907

llvm/lib/Target/WebAssembly/WebAssemblyFastISel.cpp

@@ -559,6 +559,8 @@ unsigned WebAssemblyFastISel::getRegForUnsignedValue(const Value *V) {
   Register VReg = getRegForValue(V);
   if (VReg == 0)
     return 0;
+  if (From == To)
+    return VReg;
   return zeroExtend(VReg, V, From, To);
 }
 
@@ -568,6 +570,8 @@ unsigned WebAssemblyFastISel::getRegForSignedValue(const Value *V) {
   Register VReg = getRegForValue(V);
   if (VReg == 0)
     return 0;
+  if (From == To)
+    return VReg;
   return signExtend(VReg, V, From, To);
 }
 

llvm/test/CodeGen/WebAssembly/suboptimal-compare.ll

@@ -0,0 +1,43 @@
+; RUN: llc < %s -fast-isel -O0 | FileCheck %s
+
+target triple = "wasm32-unknown-unknown"
+
+; CHECK-LABEL: gh_80053:                               # @gh_80053
+; CHECK-NEXT: .functype	gh_80053 (i32) -> (i32)
+; CHECK-NEXT: .local  	i32, i32, i32, i32, i32, i32
+; CHECK:      i32.const	0
+; CHECK-NEXT: local.set	1
+; CHECK-NEXT: local.get	0
+; CHECK-NEXT: local.get	1
+; CHECK-NEXT: i32.eq  
+; CHECK-NEXT: local.set	2
+; CHECK-NEXT: i32.const	1
+; CHECK-NEXT: local.set	3
+; CHECK-NEXT: local.get	2
+; CHECK-NEXT: local.get	3
+; CHECK-NEXT: i32.and 
+; CHECK-NEXT: local.set	4
+; CHECK-NEXT: block   	
+; CHECK-NEXT:   local.get	4
+; CHECK-NEXT:   i32.eqz
+; CHECK-NEXT:   br_if   	0                               # 0: down to label0
+; CHECK:        i32.const	0
+; CHECK-NEXT:   local.set	5
+; CHECK-NEXT:   local.get	5
+; CHECK-NEXT:   return
+; CHECK-NEXT: .LBB0_2:                                # %BB03
+; CHECK-NEXT: end_block                               # label0:
+; CHECK-NEXT: i32.const	1
+; CHECK-NEXT: local.set	6
+; CHECK-NEXT: local.get	6
+; CHECK-NEXT: return
+; CHECK-NEXT: end_function
+define i1 @gh_80053(ptr) {
+BB01:
+    %eq = icmp eq ptr %0, null
+    br i1 %eq, label %BB02, label %BB03
+BB02:
+    ret i1 0
+BB03:
+    ret i1 1
+}