[llvm] [Mips] Fix missing sign extension in expansion of sub-word atomic max (PR #77072)

[Craig Topper via llvm-commits]

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@@ -980,6 +1049,55 @@ define i16 @test_max_16(ptr nocapture %ptr, i16 signext %val) {
 ; MMR6-NEXT:    sync
 ; MMR6-NEXT:    addiu $sp, $sp, 8
 ; MMR6-NEXT:    jrc $ra
+;
+; MIPS32-LABEL: test_max_16:
+; MIPS32:       # %bb.0: # %entry
+; MIPS32-NEXT:    addiu $sp, $sp, -8
+; MIPS32-NEXT:    .cfi_def_cfa_offset 8
+; MIPS32-NEXT:    # kill: def $at killed $a1
+; MIPS32-NEXT:    sync
+; MIPS32-NEXT:    addiu $1, $zero, -4
+; MIPS32-NEXT:    and $6, $4, $1
+; MIPS32-NEXT:    andi $1, $4, 3
+; MIPS32-NEXT:    sll $10, $1, 3
+; MIPS32-NEXT:    ori $1, $zero, 65535
+; MIPS32-NEXT:    sllv $8, $1, $10
+; MIPS32-NEXT:    nor $9, $zero, $8
+; MIPS32-NEXT:    sllv $7, $5, $10
+; MIPS32-NEXT:  $BB4_1: # %entry
+; MIPS32-NEXT:    # =>This Inner Loop Header: Depth=1
+; MIPS32-NEXT:    ll $2, 0($6)
+; MIPS32-NEXT:    and $2, $2, $8
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topperc wrote:

Are these `and` instructions needed? The srav will move the 16 bit chunk to the LSBs, then the sll+sra pair will fill the upper 16 bits with the sign of the lower 16 bits. Doesn't seem like anything needs ANDed before hand.

https://github.com/llvm/llvm-project/pull/77072